As a person who has run the gamut in terms of weight, my major concern would be about their health. A person who puts on a lot of weight in a relatively short amount of time usually has underlying issues other than just the overeating. There could be depression, medical issues or similar.

I would not add on to these issues by putting them down, but would definitely start a conversation about what is going on and how I could help the situation.

In terms of being attracted to someone heavier set, I'm a girl, so I have no issue with bigger built men (and this is generally more accepted in society anyhow), but I would never date someone just based on their looks so it would not initially factor into anything.

If you're just casually dating someone or hooking up, then yes, it could easily have an effect on things because the major draw to that is physical attraction, but I'm out of that phase in my life. YMMV.