I've always felt that the numbers were probably more or less quite similar with regards to Marines, my understanding of the Heresy led me to believe that the Legions split more or less 50/50. Now yes it is true that Legions are bigger than Chapters, but then there are far fewer Legions than Chapters. The Chapters are, afterall, simply the result of a policy of power decentralisation that came about after the Heresy.
The intervening period between then and now, the Long War, has seen the Legions fragment somewhat and I doubt there is any uniform unit strength any more at any given moment the fortunes of war govern the numbers of any one Legion ranging from potentially thousands in the case of the original Traitor Legions to a mere few dozen when it comes to the many splintered off-shoots that make up the uncountable Warbands.
Recruitment wise I'd say that the Imperium has the edge, since they're well supplied and most Chapters can do it from a place of relative safety but the Lords of Terra place quite a lot of emphasis on Guilleman's 'suggestion' and Chapters, like the constantly crusading Black Templars, have to justify going over the limit. That rule rein's in their pace of recruitment I think. As for the Legion's, their recruitment can be a lot more aggressive, it is more likely however dogged by many more concerns, they have already been stated (safety, success rates, logistics etc) and I could see these problems being more than enough to slow down recruitment to near Loyalist levels, despite their best efforts.
And just to quickly address the point about CSM invasions I think it's also important to remember the fractious nature of the Chaos forces, a Chaos invasion would more than likely involve millions upon millions of renegades, mercenaries and slaves but even if an entire Legion committed its forces they should still only make up a fraction of the numbers required to take control of a world. The days of seeing an entire Legion arrayed on one battlefield are few and far between, the chances are a Legion's various Companies can be better used against particular targets dotted all over the world. Multiple Legions working together is also the stuff of nightmare, and while possible, mercifully rare.