The women in the office are adults - it is entirely up to them what they eat. Yes, it is harder when 'treats' are brought in and especially if they are 'pushed' onto you, rather than just put somewhere commual but none the less, the woman is not responsible for the other colleagues putting on weight - she's just not.
The OP said Anyway, a woman started work in my department and she is MASSIVE, really unhealthily obese
There was absolutely no need for the and she is MASSIVE - really unhealthily obese had it covered.
All this seems to be happening because the big girl is determined to feed everybody up and make them as big as her.
The OP is not a psychologist & is not privy to why the woman does this. The vast majority of people who act like this have come from backgrounds where food (and love) is in short supply - some even actually starved as children or other emotionally abusive situations.
They go out every single day (and I mean every day) to eat fast food at lunch time
She is also not responsible for a group of other women eating out at lunchtime everyday - or where they eat out. They are adults, not children.
...the big girl is getting really nasty about this and taking it as a personal snub, to the extent that my boss has told me she has complained to her that I am not working as one of the team and am removing myself from the teams group by refusing to take part in the lunches out etc
Then do what another poster did and explain why you, as an adult, are chosing not to.
they wouldn't really talk to me because they seemed to take the attitude that if I wasn't good enough for them the rest of the week then they didn't want me there on one day. They seem to be equating my refusal to pig out with snobbery and thinking that I'm better than them but I really don't care what they do, I just prefer to be healthier
So why is the OP blaming the one woman? They didn't talk to her, they made her feel left out?
The big girl in particular seems to be getting really angry about this, she really seems to have a mission to feed everybody up, I think it makes her feel better about her own weight to have other people around her be fat and having someone lose weight reminds her of her own shortcomings and frustrates her
There is an awful lot of assumption there. None of it pleasant.
So am IBU, am I alienating myself from the team for the sake of something that's not worth it, should I just join in? Or am I right not to join in with the pigging out and stand my ground even though it's making my life uncomfortable. I feel like I can't tackle her direct about this as acknowledging the fact that she is big would be hurtful and make me look like a cow
No, just act like an adult. Suggest healty places to go for lunch. Look for other reasons why you are perhaphs being left out after returning from maternity leave. Perhaphs look at if you are actually being made to feel this way or if it is another 'returning to work' issue...
But don't post nasty crap on the internet making assumptions about a women who you barely know.