Do you honestly think the iPhone would be less successful if it was broken up into a separate company from the mac business? And the complaints aren't really based on Apple's size, but its market success.
The sucess of iPhone/mac/icloud doesn't really factor into it (some of them will shrink and some will do better as they're not subsidizing other parts of Apple or can canibalize other parts of Apple's market). You'd be right in asserting the success of Apple is a factor, as those who are arguing for such a seperation would say, it's making outsized profits by not competing fairly in several of the markets it's in.