The proper response to that is not to excuse the behavior on basis of ignorance. I suspect that if a guest in your home started offering random advice on your clothes, makeup and home decor while exclaiming that it's a shame that bad taste is so prevelent, no one would suggest you excuse her behavior.
Somehow in our current "nanny" culture, if it somehow touches on other's health, it's society's problem and therefore normal social boundaries don't apply.
Bull. Weight and appearance are deeply personal issues, strict social boundaries should apply even more when discussing them. That woman was so far off base that she can't see the base from where she's standing.
To to OP, I am sure you were too taken aback to say anything, but I kind of wish you had let her know how inappropriate and offensive her comments were, because I suspect until someone does, she'll continue to offend, beleiving monitoring other's weight and suitability to their horses is legimately her concern.
In any case, good luck on your fitness journey and keep us posted on your progress!