I understand your sensitivity to being called "fat" - it's not nice. And it's horrible having a history of being bullied for it.
But in this case these people were not necessarily bullying you. They were making an observation to each other during a conversation that they thought was private. There was no bullying happening.
So the question is (and this question is for you - you don't have to answer them here on MN: 1) Are you, objectively speaking, fat? Put another way: from a BMI standpoint are you considered overweight/obese based on your height/bodyweight?? (there are other more specific ways of measuring these things, but given BMI is a starting point measure used by the NHS for various things and is a test that is easy to measure at home etc it's useful for this purpose); 2) Did your partner, in the past, say to his parents, or give his parents reason to believe, that he did not like/did not fancy/disliked the notion of dating/being in a romantic relationship with a woman who happens to be "fat"?
If the answer to those questions is "yes" then their conversation was an observational one. They were making an observation about their own child's changing decisions/preferences.
Your own comment suggests that you believe you are "fat" - and that they were not incorrect when they made the comparison statement. So, objectively speaking, this isn't a case of someone making an incorrect observation.
The situation would be very different if they had approached their son and said "we don't like the fact you are dating a woman who is fat", or "what do you see in her? We don't understand why you find her attractive..." etc etc. But they did not do that. They were having a private conversation, and making an observation about their son's changing preferences/likes/dislikes. If anything the person to be confused about is your partner, not his parents. For all we know they were making the commentary in such a way as to be glad that their son had grown up a bit and was now finding people attractive regardless of their body weight.
It's understandable that you had a bit of a gut reaction to the feeling that people were talking about your weight. But ask yourself whether the reaction was actually to what those people were saying, or more to the memory of the bullying you experienced in the past? It's probably more likely the latter. You assumed that they were being negative about you. When actually, looked at rationally, all they were doing was making a correct observation about you, and then comparing it to something they know about their son's former (perhaps even current) preferences. Cutting off all contact with parents for a private conversation they never intended to be overheard, and also which actually doesn't seem bothersome (based on the content of that conversation only) seems a big time overreaction.
A better way to handle it would be for you and your husband to talk about why you were so upset. What about it made you upset. And for there to be a calm conversation with his parents in which he explained (after all they were talking about him and his preferences) that either their recollection was wrong, or that indeed he used to not like fat people - but that now he has matured he's realised that was not actually his viewpoint. He could also say that you find conversations about your weight to be difficult, not least because you were bullied when you were little. And that he wants to be very clear that any negative discussions about your weight are not acceptable at all - and more importantly they aren't kind, and that it's easy for an observation to be misunderstood as a negative criticism.