I've taken my SF2 hucard apart. The bump isn't for the extra memory, it's that the black card part on the hucard, is a little longer than normal and they wanted a typical label area, so they used a raised bump to do that.

As far as why SF2 games are so large, a big part of the reason is all the unique frames of animation. On typical console video games, there's a lot of redundant and duplicate parts of enemies and their frames. Another big reason why SF2 carts are so big, is that all the sprite animation is uncompressed (compare this with other console games that have everything compressed). The reason for the uncompressed frames, is so there is enough time to update the frame to vram. If the frame was compressed, it would require X amount of time to decompress it into ram, then send it to vram during vblank. The Genesis, PCE, SNES don't have enough ram on the CPU side to cache (decompress) all the frames needed for a match, either. So you end up with uncompressed sprite frames that take up the majority of the rom. Why do I know this? Because I've looked into the roms to see how they were put together.