EXP

In Pokemon, as far as I know, the amount of EXP gained depends on 3 things. 
1. The level of wild Pokemon
2. The wild Pokemon's evolutionary stage.
3. Just the Pokemon you're battling against.
In other words, a level 56 Poliwhirl will give less EXP than a level 56 Poliwrath. Say you're surfing, surfing and surfing some more, and you run into that level 56 Poliwhirl and battle it with your level 23 Marill. Your Marill somehow manages to beat the Poliwhirl and it's EXP bar fills seven eighths of the way up. You finish the battle and switch your first Pokemon to your level 63 Flaffy. Then you run into another level 56 Poliwhirl. You beat it and your EXP bar only goes up one eighth of the way. That doesn't mean that Flaffy received less EXP, but rather, it needs more EXP to level up. The EXP bar never changes, so it would only be natural that the increments that it fills up by changes. Of course this might not be true in the case of legendaries, but I don't battle them on a daily basis. The level or type of your Pokemon doesn't change the amount of EXP that you gain. It's just that the individual Pokemon will make it appear as if it gains more or less EXP. I know that Ralts is one of the more patience requiring ones and most bug Pokemon are easy to level up. 


I'm not sure if there's a more scientific reason than just because behind it, but some Pokemon give more EXP when defeated than others. I noticed it when training my Pokemon to defeat Lance, but it never really registered more than 'Donphan Donphan Donphan Donphan, NOOOOOOOOOOOOOOOOOOOOOOO! It's an Onix... Why not at least a Graveler, or Golbat? *sniffle*' It actually occurred to me when going through Granite Cave, how I ran away from Zubats because they didn't give very much EXP and I always hoped for Makuhitas.