Not an RPG, but Cairn actually has this. Your bag items have physics, and there’s a bit of squishing allowed, so you can try to fill holes with items that don’t quite fit. You can also shake the bag to try and get items to settle into gaps. I spent so much time shaking my bag and trying to jam items in the smallest of gaps to squeeze everything together in hopes of fitting one more item. Great game in other aspects as well.