Yes, I have seen Marshall guitar cabs and Mesas side by side. The latter are bigger. As for the amps themselves, I honestly prefer the Marshalls. Wasn't that impressed with anything but the clean channel on the Mesas I've tried.
Make something as good as those three Marshall amps and you don't have to make anything else. Don't get me wrong, my D-180 is awesome, but if that model didn't exist, It would be difficult for me to decide between a Major and a 400 or a VBA400 and a 400+, if money were no object. As it is, the Mesa stuff I think is awesome is all discontinued, same as Marshall (unless they're still making the VBA400, which case they currently have an edge, for me). Mesa's current selection is all right, good amps. But I wouldn't take any of them over those three Marshall models.
Similarly, Ampeg hasn't made very many models I like either, yet I consider them one of the main factors in preferring US over British bass amps. They made the B-15. They made the SVT. They made the V4(B and BH), and the SVT-II (and 2pro). What more is needed?
Sunn hasn't made all that many models of bass amplifiers. 200s, 2000s, Beta, Concert, Coliseum, 300T. But they're undeniably awesome. You don't need a whole lot of variety if you make a couple things that are amazing.