Please don’t shoot me, I am not making a personal statement – only stating the perception of society.
There are people who are fat from their youth – they usually get a pass as they get older. But people who gain weight after their twenties are seen as “letting themselves go”. This carries with it the perception of laziness, or not valuing yourself, or an inability to control one’s eating habits. Because of these perceptions, people act differently toward those who gain weight.
Sure, we all know that being fat does not automatically mean you are lazy or that you eat too much. There are tons of reasons people gain weight. But slender/muscular and symmetrical are the epitome of beauty in many societies, and each person gets compared to the beauty standard and tends to receive attention accordingly.
I know exactly what you are talking about. I had an athletic build (soccer-softball player), and weighed 135lbs. I got depressed, and over 3 years I gained 30lbs. I felt horrible, and I certainly never got the attention I used to get. It was obvious.
So when I got over my blues I lost the weight and things are normal again.