This is a tricky situation, because nobody can deny the obvious, and because children can be so incautiously honest. Your daughter really shouldn't have to apologize for either telling the truth or for not yet knowing that every truth doesn't need to be reported out loud. And you, technically, shouldn't have to apologize for not knowing your daughter would be so awkwardly honest.
I think the best you can do is to either talk to the large woman or send her a card and tell her that you (and your daughter, if this is true) feel bad that HER feelings were hurt. If you know the woman personally, you could go on to tell her what you value about your acquaintance with her, and that you hope your relationship is strong enough to weather this awkward event. Then leave it up to her – she may need some time to nurse her feelings.
I would NOT say "I feel bad that my child HURT YOUR FEELINGS," because in truth, it is the woman's own self-judgement that has hurt her feelings. She doesn't like having the truth pointed out to her. It's not your fault or your daughter's that the woman feels bad about her weight.
I say all this as an older woman, once svelte, who has developed a big tummy. I'm not delighted with it, but that's nobody else's fault. If anybody observes that I have no waistline, or could usefully lose some weight, or anything else that is true, I simply agree with them. I don't have to defend myself against the truth. In fact, my grandson likes to drum on my "big round belly." That's cool with me!