I guess it depends on how big you're talking, e.g morbidly obese will certainly be harder as it limits the relationship and the things you can do a lot.
I don't have an issue if a guy is overweight, I see physical appearance as something fleeting. Intelligence is what gets me going, and sense of humour, kindness, on the long run.
My long term boyfriend is 20kg overweight, I still find him hot. There are so many things to find attractive in a man: pretty hands, arms, kind eyes, nice back, shoulders, etc.
I think the biggest struggle of being overweight when the other person is skinny is the self confidence. My boyfriend has been plagued for this his entire life, his weight makes him feel ugly and hot all the time so he is uncomfortable. He has tried several times to lose weight but can never reach the ideal weight. Now he is trying again and I'm supporting him.
Personally, my only concern with someone being under or overweight is health. You love someone, you don't want them to be at risk of developing a serious illness and dying.