Assuming I was, or was related to, a stingy restaurant owner (I am not). What would be the most optimal way of skimping people on fries?

I'm realizing this is basically the McDonald's fry container. Narrow on the bottom, but wide at the top, fanning the fries out so the maximum amount are displayed. Low cut top to show the fries' cleavage. The back is higher to support these fanned out fries. More of a stuffed envelope shape than a box, which takes up the same amount of space, visually, but fits fewer fries. The bottom is also rounded up to create the illusion of there being more room on the inside. Everything works together to deceive you.

Potatoes are cheap. Rent, labor, and sanitation are expensive. So open your restaurant in a shitty building, pay your workers crap, and let the place get nasty. Then when the disgruntled employees torch the place to escape the rats, bill it all to insurance.

Do one of those cone things, but have your “shtick” be that you only serve half and half waffle fries and regular fries. Then always put a waffle fry horizontally halfway down the cone so that it looks like it’s full. No one will complain because it’s cool and quirky that you also serve waffle fries, plus it looks big and boisterous that it comes in a huge 20” tall cone.

Waffle fries in a sleeve thatsjust larger than the fries and only open on one end. When you get it it looks overflowing with fries but really there's maybe 5 - 7

Red Robin offers unlimited fries, but the service is so bad that you only ever get one (small) helping. Viola!

Thinner the better. It creates more places for air. Same for curly fries. Even a slightly bent normal fry hogs more space than a straight one.

Curly fries are probably your The Hypothetical Cheapskate Restauranteur's best bet. Lots of volume.

you know how some glass bottles have that rounded pyramid-like shape in the bottom? start with that - it'll reduce the amount of fries that can fit in the container.

next up, take a page from chips packagers & sell fries by volume, not weight.

finally, claim that the fries are hand picked by indigenous Peruvians with a multi-thousand year history of selecting specific potatoes for specific traits.

Bonus: craft your own ketchup, mustard, and aioli (mayo) and sell it by volume (like printer ink manufacturers do).

Invest in an air fryer. These are used to fry foods while also inserting air, puffing up the surface volume of the food and making it appear bigger.

shittier burger and use some of the savings for way more fries

Those spiral ones. They get misshapen when they fry and take up more space, but they're good and kinda unique so people will buy them anyway.

the chip basically. twist them up in little fry bags is how I would package them.

You’re doing it wrong. You want more fries, to distract from the thinner burger. Fries are cheaper than meat. They eat the fries and burger and are still full, it seems like a bargain. Also see: fluffy bun.