The slider does in fact work. Yes, any piece of armor you wear will hide any difference it makes, but you'll know they're there.

Personally, a female lala with a super-sized bust would make as much sense and look as appealing as a Roe with a shrunken head, or a Highlander with huge disproportionate hands. It would be like applying 007 Goldeneye's DK mode to Eorzea; yeah people would laugh for a week or two, but then it would just get old, fast.