Why is it that when any young-ish sailor moon character becomes evil they instantly age twenty years and wear really revealing attire?

Let's take Chibi-Usa as an example. She's what, three and we only really see her in a Jūban Elementary uniform. But as soon as she turns evil and becomes Black Lady, she ages about twenty years and wears a really revealing dress? That makes NO plausible sense to me.

Then with Mistress Nine, Hotaru's hair gains huge extensions and she is suddenly really strong?

SOMEONE EXPLAIN.