100% this. and sorry in advance, but i might hit some nerves. Your physical appearances are the first thing that are going to be judged. if you're a bigger gal, and you don't like that some people don't like bigger people, then understand that you have 2 options.
*1 accept that you love yourself for who you are, and that other people's opinions and criticism's are respectably theirs and SHOULD NOT affect you(Prolly still will tho).
*2 decide that it bothers you enough that you will change your lifestyle to incorporate a more socially acceptable size/lifestyle, and then begin working towards it.
Now, I don't want to begin preaching or anything like that, but /r/fitness has everything you need to change your body shape and size if option 2 seems appropriate. But understand, I think that the goal for everybody is #1. Accept yourself. It does sound like the person in question was honest enough to share her real opinions of you, and also was not under the impression that you were aware of her talking.
Story time: My best friend, we'll call him T.J, floats between 330-370 lbs. He is a big dude, 6'2 all around huge, dude looks like a fuckin bear. I love him. But! He knows full well that he fat as fuck and big as shit. He knows that at the weight he is at, he will die young. Its just not sustainable. (I'm in no way saying OP is this big, however I do know that US 18 in womens is on the bigger size..) It's common knowledge that there is a preference in our society to accept the thinner, leaner, skinnier people. For a plethora of reasons, like perception of health, motivation, etc, this is the conclusion we have collectively come too. I personally believe it is right, but I do not choose to cast stones against those people, Like T.J, who decide option # 1.
TL;DR: Just remember, society is not going to change, your are fine how you are, unless of course you are uncomfortable to the point that somebody talking about your weight/size/physical appearance bothers you, in which case you are no longer fine, and should begin a body transformation regimen, to fix you're problem once and for all.