You're completely right. Assets and large file-sizes are allowed by the rules.
It just becomes ethically questionable to post asset flips and games that hog storage space to game jams, especially when people are trying to download a lot of games in a short time.
The purpose of a game jam is to test your skills as a developer and learn, not to test your credit card, thats why its called a game jam and not a programmer jam. I spent half a week modeling and creating assets for my game, and it cost me a lot of gameplay potential, but I learnt so much about modeling, texturing, map building and sound design and ill see the rewards of that work in any future games I make.
Again, assets are completely allowed, and I have no problem with people using them to speed up the development of their game, but having games be almost entirely made of assets is a bit of a not cool thing :(