Wing loading. After all, the bigger a plane gets the heavier it gets, but it does not necessarily fly heavier. You can really ignore the weight in relation to ther models in the case of planes like the UCD3D and the flip. For instance, do you actually have a .40 on your flip? I would use a .72 or 100 on it for sure. For some reason though the guys that manufacture these planes insist on using bogus engine sizing numbers. Look at the UCD60, I wouldnt use less than a 100 on it. The UCD for, .72. Funtana .40, .72. Flip .40, 72-100... you get the idea.

-Ian