I completely understand how this is a big deal. When you commit your life to someone you not only love them but want reciprocal trust,respect, pride etc. When faced with an unnecessary lie it makes you think their behaviour is a bit shabby and you respect them less.
I've been with DH for 15 years and a lie he told has stalked our marriage. On our first ever date we were talking about our lives. I already knew he was divorced. I asked him if anyone was involved in his divorce. I remember him looking deeply into my eyes and saying very affirmatively "Categorically not! ". Naively I completely believed him.
Of course, years later when we had bought a joint house etc when drunk he admitted he had been having an emotional affair with a married colleague, which turned into a real affair after he walked out from DW1.
This lie has haunted our marriage. I felt tricked into getting involved with him. Of course I then looked for, and found, evidence of other exaggerations or minimisations. The fact of a lie like this undermines your respect and trust. It also makes you hyper vigilant.
I do wonder ( and I'll be flamed for this) if it's a more male thing? Could they be more primed with each other to exaggerate their prowess in a testosteroney competitive way. So living at home with your Mum at Uni is not buffalo hunter gather behaviour. So to increase your prowess you exaggerate. 1950s thinking I know and I don't want it to be that.
I think it's all tied in with ego, self esteem etc.
Perhaps all you can do OP is concentrate on your response and on his positives and forgiveness... but then I also think why should we have to! Are they men or mice!