I have always been a fairly average size through childhood and teens and a size 10 or 12 til my late 20's when I went up to 12/14.
I had my DD and also developed an underactive thyroid which went undetected at first and led to me gaining weight. I am now a size 16/18. My mum has been very slim her whole life.
She is constantly asking me when I'll be losing weight, what I'm eating and generally keeping on at me. Things came to a head earlier today. My stbxh left me 18 months ago for someone he worked with. My DD 6 has just spent her first weekend at the OW's flat. My mum said it must have been hard for me and did it affect what I ate today 
She then came across a photo of the younger, slimmer, child free OW on facebook and said 'she has a lovely figure, if my husband had left me for someone else I would have worked hard to be as thin as her'. She then got annoyed when i got upset by that!
I get her point. I look better slimmer and feel better about myself but I don't let it hold me back. Despite my husband leaving me for OW I've tried not to let it ruin my self esteem. I go on dates, out with friends, coach a sports discipline a few times a week and dress to suit my size.
I said to her that it's her that has a problem with my weight and not me. Her reply was that 'she doesn't like looking at me like this' 
I have to say she has been a great support to me and DD and is always there for us but I'm just not sure how to get pass this issue that has been hanging around for the last 6 or 7 years 