You stated in another response that your app was targeted at 2-8 year olds. I'm not sure that target audience is a good base of comparison between Android and iOS.

My 7 year old daughter has an iPod and she get's iTunes points for gifts. I am NOT however, going to give her an Android Phone. Until there is an Android version of the iPod and a way to buy gift cards for market purchases, I don't see the under ~15 market being lucrative in the Android space.

EDIT: I would also bet that 100% of your sales on the Amazon market are from Kindle Fire users. THOSE are the only small form factor Android devices that are cheap enough to give to children, thus your success there.