Why is it relatively easier for me to gain weight vs other CFers?

So I saw a post that said someone was surprised they gained 15 lb. in like 5 years or something and weighing the most they have in their life (which was a moderately low weight). For me, it's not that hard to gain weight, it's harder to gain weight for me than normal people, but it seems easier to gain weight than other CFers. Why is this? I'm 18 and I'm 5 foot 9 inches and I weigh 167 lb. I just gained 10 lb. in like 5-6 months.
Source: Cystic Fibrosis Teenagers and Young People Forum - Category: Respiratory Medicine Authors: Tags: Teenagers and Young People Source Type: forums