A to B magazine found the brompton to be 85 liters in volume (August 2006 issue) when folded compared to 188 liters for a 2008 Mezzo (July 2008).
Given that a big part of having a folding bike in the first place is fitting in small places, it seems that the two bikes aren't really comparable. (And I speak as a Birdy owner :-)
That doesn't mean a Mezzo isn't a good bike, just that it really isn't in the same category as a Brompton.