Slave girl has become significantly shorter from the transformation. Her change in height is now more apparent to her because she has someone to compare to. I don't really know how to make it clearer in the comic without "oh now i am teh little girl" type narration.
Also the mistress is standing straight and probably in the heels she was wearing earlier while the slave girl's knees are bent (in the second frame at least). It's really not that difficult to understand.
It's pretty obvious to me what you were saying in regards to her height. I suspect some readers were too busy getting off to actually read the dialogue.
