As someone who, if anything, wants to gain weight, this has the inverse effect for me. Peanut butter is great because it's so dense with calories. (I try to avoid sugar though, but I don't think peanut butter has much added sugar, but I'll have to check next time I have some.)

I know I'm in the minority though. I'm a 6ft tall man who weighs ~125-130lb. I was stuck at 120 for a decade or more, and only recently have put on a couple more pounds. I don't have an eating disorder or anything, but I just struggle to put on weight, so my serving sizes personally for peanut butter are way higher than average.