The planes that hit the twin towers were bigger, going faster, and had more fuel.
The twin towers themselves were also built with a different skyscraper design at well that used fewer steel beams. I don't remember what the names of the skyscraper design types were but I remember a 9-11 history channel program going into it.
I may be wrong but I recall the twin towers had a central spine that was the load bearing component like a tree or something. Older buildings had a frame and load bearing exterior with a soft, gooey center.
I remember it being explained as the twin towers "hung" somehow, so the central spine makes sense.
The older buildings were basically just steel beams like you see in cartoons. Lots and lots and lots of steel basically in cubes from what I recall. So there was just a lot more to catch the load. In some sense they were overbuilt.