I just wanted to share my little experience with you guys, to get it off my chest.
I live in the UK and on Sunday it was Mother's Day, so I invited my mum round, and my boyfriend and I spent 4 hours cooking her a fantastic meal.
Anyhow, I have one of those mothers who is overweight herself, yet thinks she's fine for her age, and spends her time telling me how fat and overweight I am. I know I'm nowhere near as thin as I used to be (or as thin as she was when she was around my age, I keep being told) but that it no excuse for what happened. Basically, at several points during the day, she started grabbing and poking at my love handles and saying "What's all this?" "Ha ha!" The worse thing is, my darling boyfriend joined in the grabbing! AT THE SAME TIME!!! I hadn't realised I had quite so much fat to go round!!!!
Then when I get upset, I get accused of having no sense of humour!!!!
Basically, it was torture! And it keeps coming back to me in flashbacks!
It doesn't help that both my boyfriend and my brother (who was also there) are tall and toned. The ironic thing is that I basically look exactly like my mum, just 30lbs lighter! But you really can't call your mother a cow
, especially on Mother's Day!

OMG that's really horrid of them.
Whatever you do, please don't let them make you think you need to be any smaller. I am really really apalled that you have to be subjected to this treatment. Stay strong.
I am shorter than you and at 126 lbs, no one would have suggested I needed to lose more weight. If it had been me, I swear, I would have done the same thing back to her. I have no compunctions about calling my family members cows if they start it. In fact, I had this very conversation with my overweight Dad when he started making comments about my step-sister putting on a few pounds (luckily he made his comments after she was gone).
And, when I HAVE had to buy bigger pants, for example, I've rationalized terribly by saying, "Oh, I just like my clothes loose and flowing". Right. A 2X is loose and flowing. A sz. 12 USED to be loose and flowing. That's a LOT of rationalizing, eh?
(Sometimes you have to have fun with this stuff; otherwise it could drive you insane, eh?)
. I know you've been with him a long time, but it sounds like a habit to me - one that it might benefit you to think about breaking. Whittlin made some awfully good points about having your significant other feel protective of you rather than being the first to jump in when you are being harrassed. And your mum WAS harrassing you; if it's a family tradition, no wonder you're so accepting of your BF's doing it as well. Ugh. If you were MY daughter (and I have three, all in their twenties) I would be giving the same advice, although I really don't have to because they are all married to men who worship the ground they walk on. (And that's what they EXPECT; I raised them to think very highly of themselves). 
but I think you get my drift...