What you just described is not a lie. It would be a lie if you were a martian having no understanding in how humans operate. The question "Does my bum look big in this?" may be a factual inquiry about the qualities of the chosen garment or may be a solicitation of a social interaction expressing support and admiration, due to the feelings of insecurity or needing external validation. Yes, I know it sounds the same, humans are complex. But calling such social interaction "a lie" just because it is not a factual exchange is IMO misunderstanding the whole process and devaluing the term "lie".