balloon frames are wider where the tire passes through the frame/fork.
A balloon tire (2.125) on a typical rim usually has no trouble fitting in a middleweight frame/fork. Using balloon fenders on a middleweight frame isn't easy though.
my gimmick...Use 3 fingers. Put your first 3 fingers in the triangle just below where Jeremy's tape measure is. Your fingers will be snug in a middleweight frame but plenty of room in a ballooner. Gary