hmm if I have to choose, it is definitely more challenging being a fat guy.
First, if you are skinny and want to grow fat, just endure for 3 months and stuff yourself with food. You will definitely grow. However, if you are fat, it is a torture to lose weight, and it's harder to lose weight fast. This is a simplistic way of answering since this is just a comment not a hub lol.
Secondly, fat people face more derision than skinny people in many cultures and societies.
So it is definitely more challenging being fat, though I won't really know since I am an ectomorph haha