The usual suspects for memory eating, IME, are code weight and large cached data structures. The $menu structure in D5 is decidedly not small, for instance.

For code weight, even in D5 you can do some nimble footwork to split off page handlers. See the way locale module handles it for a working if a bit ugly mechanism.

For large data structures, that's frequently cached data. While the look up in a giant array is far faster than hitting the database, hitting the database has less of a memory overhead if you're only pulling a few records. I have no idea what the code looks like now in UC, but that would be a place to look to change the trade-off.