The one and ONLY person to judge yourself against
As human beings, it seems to be in-built into our nature to compare ourselves to others. To rank what we have against what another possesses, in whatever capacity - 'Oh, that person has a really nice car', 'She can sing so much better than I can', 'I wish my hair was naturally curly like that'. This is never truer than when we compare our bodies to someone else. 'I wish I had legs like hers' or 'I want to be skinny like Keira Knightley', 'Why can't I look as good as . . . . '
I have shocking news for you people and it boils down to this - You are YOU!! Although we all share certain similarities with regards to gender and we're all made up from the same stuff (blood, bone, muscle etc) that's pretty much where it ends, everything else about you is unique. Your particular hair colour and texture along with your eye colour, the shape of your head, how you stand, walk, drink your tea etc. Every aspect of what makes up who you are body-wise, is down to a mix of genetics and what you've done in life up until this moment.
Comparing yourself to another is fruitless and is only ever going to be a negative thing. No matter how hard you try or how much you train, you will never look like X (fill in name of person whose body you covert) because you are not them. This doesn't mean that you should give up because you're never going to achieve it. What it means is, what you should actually be aiming at, is the best & healthiest possible version of YOU! If you absolutely must make a comparison then the only comparison you should make is with yourself - 'Am I in better shape than I was a month ago?', 'Do I feel more energised?', 'Look how much stronger I am than I used to be'
Just as it's not worth you comparing yourself to someone you perceive to have a better body shape than you, it is equally unproductive to compare yourself with someone who appears to be in a worse state than yourself. 'Well, I may be a bit overweight but at least I'm not as fat as she is' or 'She may be thin but she doesn't have as well defined muscles as I do'. As I have said, you should not be putting yourself down because you don't have certain attributes that someone else does and neither should you justify your body image by being negative about another persons. It doesn’t get you any further along your path to looking and feeling better, in fact, if anything, it may stall you. People generally use this type of mentality as a way of justifying not doing anything about their own circumstances. Feeling that, as long as they can find someone in worse shape than them, that somehow puts them in a superior position. This, of course, is just self delusional bollocks and will stop you from truly achieving your desired goals.
So, stop comparing yourself to others. Strive to be the best YOU can be and let others do the same for themselves.