The way I always imaged it- Callie gives off a young playful appearance...while Marie tends to look a bit more mature-like and slightly older. Not to say Callie isn't mature herself, of course. But you definitely wouldn't disagree at saying she's more of a hyperactive goofball of the two. So I think that's why she has a younger vibe to her as opposed to Marie, which can effect their appearances in a sense.
I find it funny because it reflects themselves in the weapons they use too...if you think about it.