It’s mostly just large. 4 crew instead of 3 and an auto-loader, and they focus on extra thick composite armor instead of a mix of that and reactive armor. And meanwhile, the t-72’s an extension of old Soviet philosophy that a smaller tank was that much harder to hit, so everything’s packed as tight as possible, while you could almost have a small cookout in the abrams.
/Edit - And as mentioned elsewhere, the abrams is stupidly heavy at this point and has even gained something like 20 tons over the years. A smaller, lighter tank has less problems crossing softer terrain and getting over rickety bridges.