I have naturally curly hair too, and have found that even diffusers make the curl in my hair smaller and kinkier as well as frizzier.
Even though I have an expensive ceramic dryer with a diffuser attachment, I still let it air dry as often as I can – often with the assistance of car fan on blast during my commute. I also open windows, have a small desk fan near my computer that helps while I check all the news in the morning and I also like to have it wind-dried by the water – which is near my house.
In the meantime use a curl enhancing product, like Curls Rock or Bumble and Bumble’s Curl Conscious and then, if you want something small and effective that won’t cost a lot, try this basic diffuser. I used this all through college and it worked better than some of the fancier kinds I’ve had.