I originally wrote it so Susan was all like "I dunno about this", and Nanase was like "you're a chicken", and Susan was all like "I'M GIVING IN TO PEER PRESSURE", and then I was all like "that's dumb", so I rewrote it to be more in character for both of them. True story!
Before anyone asks or points out anything, yes, it's still growing and shrinking. Tedd's referring to the difference between a change in height and a change in scale. It's like the difference between Susan becoming Nanase's height with adjusted proportions, and Susan becoming Nanase's height with the same proportions. They're both shrinking, but the size difference in volume would be more dramatic with the latter than the former.