At its crudest, it sounds like a school playground invitation to stick with the big bullies rather than be part of the group of smaller children suffering the bullying. Only slightly less crudely, it implies that bigger states can and should get their own way more often and/or an unfair share of resources. If it doesn’t mean any of those things, then it’s surely a meaningless phrase.
As a statement of the way things actually work in the world, it’s difficult to refute the argument. In practice, might usually is “right”; but is it the way things should work? Is it the way we want things to be?
I certainly don’t, and I’m not convinced that maintaining the strength of the bullies – which is what Cameron is effectively proposing – is the best way of tackling bullying. It’s not the advice which Cameron would give to children who were being bullied at school (or at least I don’t think it is!), so why do so when it comes to the big school of international politics?