This system is an aberration, the amount of experience needed to gain a level has always been to same regardless of the evolution stage the pokemon is in. So a squirtle and a wartortle need the same amount of exp to reach level 20 from level 19 in the originals games. The generation V exp system was different but just for the exp gain in combat. In gen V if a pokemon win against an pokemon with a lower level, the exp will be reduced proportionally to the lvl difference. And that's the only change.
The way experience is handled actually is really unhealthy for the game. I understand that pokemon revolution online is mean to be harder than the original games but this doesn't seem to be the right way to do it. I should be keeping my ghastly until level 98 if i want to exp faster ? Just seems like nonsense and in no way faithful to the original.
I recommend everyone who doubt what i'm saying to check the bulbapedia article about the experience system in the pokemon games.