6th Gen altaria or my imagination

Is it my imagination or does altaria lvl as fast as rabbits? All my other pokemon are like... 15 levels behind my almost 100 altaria and i feel like i dont use it that much.

It's not your imagination, actually! All Pokemon have a rate at which they gain experience. You can see it on Serebii, actually, it's right under where the abilities are listed on the Pokedex page. It says Altaria has "erratic" growth, and it only need 600,000 EXP points to reach level 100, while Blaziken needs way more and is classified as "medium slow." Not sure what other mons you are using but chances are that Altaria is faster as well!

wierd. I would have figured with the amount of strength altaria has that it would level slowly, but its just a speed machine.

I understand you here since my Altaria often leveled up higher than the rest of my team in my Alpha Sapphire main sory run, and being in a very fast EXP group explains it. As Altairis said, the EXP group is what determines how fast the Pokemon level up so don't be surprised if some of your Pokemon end up at a higher level most of the time, because it meant they were likely in a fast-growing EXP group.