the very point of this is to lose some the old painful browsers (IE6/7/8) to make code much better. almost all work in cross-browser stuff is IE6-8. think of jQuery mainly as a bugfix for old IE's.
jQuery only lost 10% of its size on full builds when it eliminated IE6-8.
There is still plenty of stuff left to handle. Naive code that only handles common success case is of course going to be small. Even if the modern browsers didn't have bugs and behaved exactly the same, you would still have most of the stuff there.
jQuery features amazing event model with endless corner cases and optimizations* handled, powerful ajax library (Think of stuff like prefilters and transports) and so on.
* For example your typical delegation implementation traverses the propagation path over and over again for each handler, while probably not even checking for simple selectors.
