Her motives might be several-fold.
She could be trying to show her dislike of you, trying to keep you 'in your place' and/or making it clear that she's still a big shot in her DS's life i.e. 'Mum's always here for you and won't let you go hungry, whatever that wife of yours does'.
She might well be trying to 'outwoman' you. If she's of a generation and upbringing that believes you show your devotion to the men (the main focus of family life, of course) by playing the martyr and shrinking back as she believes all 'good' women should. She has no interest in competing with the men to protect her status as she sees her position as a uniquely female/matriarchal one that they couldn't possibly (or wouldn't want to) try to usurp.
I wonder if she's deliberately trying to starve you out and goad you into asking for more food so that she can win 'good subservient woman points' by feigning extreme surprise that you'd feel the need to eat any more; she's a woman too and she manages just fine.There would be a (probably unspoken) implication that you've committed a terrible faux pas and have utterly failed at 'womanning'. After all, we need to maintain our weight and figure to keep our men interested, don't we?
She's ridiculous, whatever her bizarre thinking is. As your DH is on your side, instead of him giving you some of his, I'd just agree to do what the DH mentioned upthread did and swap plates (make sure you get him to do it immediately himself, so she can't accuse you of, ironically, controlling him). Alternatively, both get up to go to the toilet or fetch something from the car etc. and then sit back down in each other's original seats.
She'll either learn that her game is up and start treating you fairly or otherwise protest, in which case get your DH to patronise her by saying "I know you're not very active, Mum, but Sofin works very hard and works up a reasonable appetite - there'll be plenty of time for serving child-sized portions in nine month's time!"