Is possible to know if market share mean more virus?

I'm having a argument with others developers about why exist more "virus" (here collect all forms of external attacks) in android vs iOS. Like with Windows vs *nix the main idea is that the bigger market share is the determinant factor in the increase of attack attempts.

I think that the main factor is how easy is carry a successfully attack, ie: Windows have more virus because was a weaker target than others.

So:

1- More numbers (population) = more attacks or 2- More weak (security) = more attacks

I acknowledge is necessary a sufficient # of targets with enough value to sustain this behaviors, but think that is possible to have a more secure system, no matter how popular and that the increase on numbers will not yield a increase on insecurity.

However, that is my intuition. Exist a solid proof?