In 2.1 (or whatever the first big patch was, after the release of BC), they reduced the experience per level for 1-60 by 30%, as in you only needed 70% as much.

Ditto for WotLK, a major patch into it, they reduced experience per level for 60 to 70th.

Factor in 19% faster leveling, from shoulder + chest heirloom items, and we only need half the experience for 1st to 70th.