Exp was level scaled.
So, if a level 5 beat a level 50, it got a crap ton of Exp.
If a level 50 beat a level 5, it got next to nothing.
Sounds decent, until you realize just how painful it made grinding. Basically, as soon as you got over the level of the Pokémon you were grinding on, you got less, and less, and less Exp.
It made getting to 100 a pain, and threw off the overall Experience curve of the game, IMO.