Why are laptop adapters so much larger than phone adapters of same power rating?

GaN is newer technology and more expensive to manufacture. It can power laptops like anything else just in a much smaller package. I have a 65W GaN charger that charges my work laptop while on the go just as quickly as the official brick I leave in the office. You just need to pair it with a USB-PD rated cable that will handle the wattage your laptop requires. In my case, I ended up getting a 240W USB-C JSAUX cable. It’s stiff but does the job.

One significant difference that has been missed here is that Laptops can and often do run on the power supply, while phones usually use the power purely to charge the battery.

It's a significant difference as the laptop needs a stable electricity supply to supply all it's components or the laptop would crash. That means not only does the brick need to dissipate heat, but it also needs to be able to deliver a stable continuous DC current. So as well as a transformer and rectifier (that together convert AC to the correct DC needed) there are smoothers and potentially capacitors to ensure a smooth continuous output even if the wall supply is janky.

If you turn off the power at the wall / unplug you often see any light on the power brick stay on - that is because of the capacitors and there is still a small amount of energy available to the laptop as it discharges.

While phones are mini computers they are usually designed to always run on the battery. Even when charging, the phone draws it's power from the battery and it's in built circuits to smooth the current; there isn't usually the redundancy in a phone to switch between different supplies in the same way as a laptop. There isn't also the expectation that they need to run off the wall continuously by users (even if users might plug their phone in and expect to continue to use it, they will find their phone shuts down if its at 0% and they push it beyond what the recharging battery can supply; a laptop would be expected to run solely on the wall not shut off).

Things are blurring now with USB C power supplies for laptops - but you will find the plug itself has more of the electronics built in or some of the functions of the power brick have moved into the laptop to reduce charger bulkiness. Look at how bulky a USB c charger plug is for a Mac - they're not simple USB chargers you'd use for a phone or tablet, they're bulky because they are also doing the smoothing and stabilisation people expect for their laptops.

Tl:Dr they're different sizes because they're doing different things. Basic chargers purely charges a battery, while laptop chargers both charge a battery and provide good quality supply to keep a laptop running optimally.

My guess is it's similar to the electric motors they put in cars vs industrial motors... they can both put out X amount of power but the industrial ones (or in this case the laptop power) can do it continuously without overheating while the other can only output "up to" that power rating for a short time without burning up.

In this case though, the laptop power probably also outputs a higher voltage as well.

Don’t forget safety. There’s a LOT of of sketchy ass gan (and non gan) chargers I wouldn’t even let in my house. Gan chargers being by so compact dont have enough space to safely insulate things.

Pro tip: eBay used laptop USB chargers are dirt cheap compared to a GaN charger + USB PD cable. I've put laptop chargers all around the house so I always have one available for my laptop, phone and Steam Deck.

Another tip: even if your laptop charger isn't USB C, you can get a converter to make it a PD USB charger for a couple of £.

Or is it simply cheaper to manufacture while being sold for the same price?

Yes, it's mainly that. Most vendors don't make their own power adaptors anyway, since there are strict requirements for power adaptors that are plugged into AC wall sockets in each country. The stuff inside the large brick is likely sourced from somewhere else, who has all those certifications worldwide. Note that it also uses standard AC power cable plugs. That large brick can be shipped worldwide by simply changing the AC cable it ships with.

The small GaN Brick is also outsourced, but since it is smaller it is probably more expensive to build. However, the AC plug is integrated into it, which means the seller has to stock physically separate units for each plug type if they want to ship worldwide.

As far as the price is concerned, this one shop is selling them all at around the same price, but I bet the MSRP on the Lenovo one is much higher, because of the brand. The parts inside the Lenovo one are probably cheaper.

In general with electrical things, the longer it will be operating at a higher usage the bigger the need for heat dissapation and more robust parts. Phones of the same power rating still have less juice, as in smaller batteries, and have less power usage overall.

While the power rating may be the same, the volume of energy is not the same. Like running water through a straw and a hose can have the same pressure but way more water is going through the hose.

Watts are part of the electrical properties along with volts and amps. Laptop chargers have the same watts and volts but more amps.

Edit: I got the calculation wrong, but the point is that more power goes through that laptop brick in a shorter amount of time and it needs to both handle the higher load and dissapate more heat.

Voltage is the same but wattage is not.
Most of your phone chargers will be 65w or less while a laptop isn't going to charge on fewer than 100w most of the time.

My uneducated guess is money.

Manufacturers likely have factories (either theirs or a contracted company's) where they can mass produce the power bricks for a low cost. Upgrading to a USB power supply doesn't offer significant benefits compared to the power brick of similar wattage, and the up-front cost of setting up a new supplier is financially unjustified. The old technology works just as well, so why change?

High power USB is still a relatively new technology. I'm sure it will proliferate, but the consumer market has a fuckton of inertia.

far different size.

Size first. Power electronics don't have the same miniaturization benefits (or expectations) for laptops as they do for phones. Some factor here is that there's just a different convention for these two different types of devices.

larger bricks that have separate mains cable which plugs into the brick

There's a manufacturing benefit having one circuit where you can just change the mains cable for length or country adaptor types. High probably the wall warts have the same ability, but it isn't as obvious if the plastic parts are press fit.

Why is that so?

Some laptops could run off of 65 w, but only the very basic types. An Alienware m18 R2 gaming laptop has a 280 w adaptor and it isn't close to the same size or price as a phone adaptor.

Same for phones. 65 w is pretty beefy for a phone charger. It will come down to the requirements of the device.

Generally speaking, a charger can have more amps than the phone or laptop requires and that's usually okay. The voltage is what you have to be careful about. 5v is kind of a common electronics safe voltage. Good chance all these "universals" chargers are 5v.

Again, something like an Alienware m18 R2 is going to require more than 5v.

Or is it simply cheaper to manufacture while being sold for the same price?

Good chance of that. Much easier to price fix on big warehouse type online marketplaces.