Tom, thanks for your reply. You are probably right, it seems likely that it is because the load factor. I do remeber reading that some light twins have a minimum weight because of Vmca requirements, but I can't seem to find that page again.
Sorry, but I can't resist this: You might have written "minimum flight WEIGHT of 3329 lbs", or "minimum flight MASS of 1513 kg", but not mix them. The Diamond/Austrian/Canadian/European use of "mass" is more "correct" than the typical American usage of "weight" (an airplane's weight changes with altitude, but it's mass remains constant - fuel burn not considered) Please take this as tongue-in-cheek, not an anal criticism on my part.
Seems like I mixed up weight and mass there. I'm not from the US, but I've done my flight training there. I'm more used to the european definitions of weight and mass.