"Does my bum look big in this?"

"Does my bum look big in this?"

How should one answer?

I think it depends what the person is really asking.

They are unlikely to what a precise analysis of the proportions of their posterior, although I suppose just perhaps they might want some friendly fashion advice.

If what they want is a bit of appreciation and encouragement, then I don’t think it’s a wicked sin to affirm that they look lovely, even if their bum is big, and it's not your duty to tell them that last bit!