No, they are not going after Android because of the quantity of them. They are targeting Android because it is an easy target. It is weak for security thanks to it's open nature and lack of control over app deployment.
There are 100's of millions of iPhones out there and people with iPhones are proven by statistics to be of higher income and bigger spenders on average (partly proven by the fact they paid for an iPhone rather than settling for a cheap or free Android). Translation: iPhone owners would be more desirable Malware targets. But they are barely targeted at all thanks to the iPhone's secure nature.
Android: open in all the wrong ways
BTW don't try to tell me iPhones can get Malware, of course they can but its very difficult for it to seed and spread.