I mean they shoved 3D Mortal Kombat and 3D Tekken into a GBA cart too.
Yes, but when we're talking "managed to fit on a single GBA cart", that's a little different from having the the technical firepower to make amazing 3D engines on the meager hardware. We're specifically talking about fitting a huge amount of art (sprites and backgrounds) into the limited storage space of a cartridge (and live game RAM, sometimes, if the game has a lot of imagery at a high framerate.)
3D Mortal Kombat and Tekken were also not
exactly 3D games. They were complex "2.5D" games, you might say, with a Mode-7 style floor that moved under the fighters and in/out/around in perspective, a wrap-around background image image (or actually two backgrounds, for parallax effect,) and then the characters were 2D sprites rendered with the ACM method. Those sprites had limited frames of animation compared to Alpha 3 and were also fairly optimized by the computer rendering in color/resolution (although I don't think GBA could have really taken advantage of image compression?), so I would guess they're actually not using as much data as a comparative SFA4 character?
That said, the complete package of movement and zooming and perspective orientation and effects and movesets added up to some worth equally worth celebrating in Tekken GBA and MK: DA GBA, if you're a fan of GBA development heroes.