I don't think one can measure how strong chaos is or isn't. It's obviously an extremely powerful force, capable of corrupting nigh everyone. Most likely every human alive feels it's alluring pull to some degree or another. But as for military? I don't think Chaos will win due to black crusades or even chaos space marines but the increasing growth of cultists festering at the roots of the imperium, killing it from the inside.

In terms of raw military might the singel strongest faction is the imperium of man, orks are stronger but splintered and fight eachother as much as anyone else and the same can be said for alot of other factions.

I'd say the "strenght" of Chaos isn't really comparable to the other powers, it's not even measurable by the same standards. Chaos is afterall powered by just that: chaos. It needs war to thrive,which is why invasions by Chaos forces are always at their strongest at the early stages when bloodshed, fear, hatred and a wide array of other related things are abundant. Then, the more Chaos conquers, the weaker it gets simply because of the lack of resistance (read: fuel). Chaos as a power can not be defeated, but neither can it win: the moment Chaos conquers/destroys everything, it will be out of fuel and pretty much implode. Chaotic power at its best, constantly seeking to destroy what's keeping it alive.

I'm pretty sure I've read/heard the theory somewhere else, so not meaning to steal it or anything. It just happens to make a lot of sense and as such I'm leaning towards this being the case.

 I wonder why noone has yet given the obvious answer: No, Chaos is not stronger. It's quicker, easier, more seductive!