people always get used to themselves. i bet even the pros say the same thing. remember when you were a kid and first started training? i was a chubby kid, some of you may have been skinny. either way if that 'kid' were to see his future self, he would think - 'damn that guy is HUGE'. but you got used to yourself and now you always think you need improvement.
at least this usually leades to something positive, you attempt to make your diet better or train harder and smarter.
something akin to this, but in a negative way, might be anorexics where they are sickly small and still think they need to lose weight. this leads to extremely poor health. and i said weight there with the anorexics, not fat, cause most anorexics through starvation have lost virtually all lean mass, and technically are obese by bodyfat percentage.
another idea: i myself, having been a chubby kid, always had an easy time gaining weight. i can gain weight in a flash. for me dieting down to get very lean is difficult. i was always a bit envious of those guys who were naturally lean and could get ripped with minimal effort. those same 'lean' guys, have told me they were always a bit envous of people like myself who could gain weight so easily. i guess people always are seeking what they dont have