Taken directly from my Bradygames Strategy Guide: "Improving Your Characters: This aspect of the game works a bit differently from past Final Fantasy games. Characters will gain exp. by defeating enemies, and then level up when they reach a predetermined amount of accumulated exp. points. However, now all that is required to go up a level is 1000 exp. points, regardless of the level you're trying to achieve. The flip side to this is that your characters' stats increase less from level to level than they have in past Final Fantasy games."

page 4 of the guide, first paragraph on the page.

~Edit~ I think this should answer your question.