Cheaper & faster development by leveraging large libraries/frameworks, but inability to automatically drop most unused parts of those libraries/frameworks. You could in theory shrink Electron way down by yoinking out tons of browser features you're not using, but there's not much incentive to do it and it'd potentially require a lot of engineering work.
Storage is cheap on a PC, it's not cheap on mobile where it's fixed and used as a model differentiator. They overcharge you so much. Oh, and they removed SD card slots from nearly all phones.
Yep. Apps are 20x bigger with no new features...that you are using.
Let's not forget that the graphics for applications has scaled with display resolution, and people generally demand a smooth modern look for their apps.
In the case of normal apps like PayPal graphics shouldn't be a huge factor since it should be vectorized and there is pretty much no graphics in apps like PayPal.