Ours is one of the very few cultures on the planet in which being called "plump" or even "fat" is considered an insult. We think if someone notices we're larger than other girls they must think we have weaker strength of character or something. But in other countries size is just size. There is no moral implication. I remember when I was in the fifth or sixth grade, I was overweight, and American kids would tease me mercilessly. But my aunts from Singapore came to visit, looked at my growing chest and hips, and said, "You must be popular with the boys."
Oh, and I just remembered: have you ever read Anne of Green Gables? Anne (with an "e"

) has a big inferiority complex because she's
skinny. She envies her best friend Diane for being, literally, "plump" and rosy-cheeked with tons of admirers writing declarations of love for her on the trees.
Also, women take it a lot more seriously than men. The other day I was complaining about my body, as usual, and my sweetie said, "You actually look very small. From the waist up you look like a freshman in college. You just carry weight in your thighs." My first reaction was to be offended, but really, he was just stating a fact. And obviously he finds me sexy anyway, and says so at least once a day.
So this is the way your coworker sees it: you're "plump," and you're pretty. There's nothing incongruent about it.