if you measured effective top tube on both bikes you would have some idea about how much longer stem you'll need on the smaller one. but I can tell you that if you're already riding a 120+mm stem on your current bike it's very unlikely that smaller one is going to work.

a bike is too small when you can't get the proper fit on a 140mm stem, or can't get proper saddle setback according to your femur length using a setback seatpost. either of those makes a frame too small. BOTH of them make a frame "way too farking small".