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".

But these lies are exactly why psychopaths lie. They start by telling the truth, and realising it doesn't work. They learn to lie. They discover that does work. They then use this technique and apply it to everything. Lies become more than the social oil that everyone uses to avoid offence, lies become a tool of manipulation.