This is a really interesting and puzzling topic in the ostomy community. In my 16+ years since joining the ranks, I've noticed that ostomates tend to either lose a lot of weight or gain it. There seems to be a very small percentage that falls in the middle of the road. There are a lot of different factors that can contribute to weight gain. Age, most commonly, being less active than you were when you were younger, and also depression and/or stress.
I was very active right up until maybe 6 months before my surgery - then I started to get very ill due to the severity of my UC, so I started dropping weight. When I came home from the hospital, I was close to 20 lbs below my weight I had been for over 15 years. Because I had been very physically active, it was very easy to resume my active lifestyle once I was fully recovered from the operation and I got my appetite and strength back.
In my 10 years or so on various different support sites, I have spoken to many and encouraged them to be more active - to not let the ostomy hold them back from leading a normal life and, most of all, don't let it rob you of being happy. My findings have been that a lot of people, especially ones that were not actively exercising or doing anything other than their housework or job that they had to do, are afraid to do any physical exercise for fear of injury - most notably ending up with a hernia - which we all know is an injury that is very difficult to recover from. I've had 4 surgeries, so even with working out for close to 30 years, and even at my high level of fitness, I'm ever so careful at the gym when core training. The only ab exercise I do is light crunches lying flat with my legs raised in an "L" position.
For the inexperienced at exercise, there are several things you can do to get in better or stay in shape that are low impact and won't put you at any more of a risk of injury than picking up around the house. Walking for 30 minutes or more is a great start. Swimming is the best low-impact total body workout you can do. Watching what you eat alone won't make you lose weight, you have to increase your cardio level/heart rate to increase your metabolism to where your body is burning up more than your body is intaking. Just because we all have a short bowel doesn't mean that you can't have a slow metabolism anymore. I used to have a fairly fast metabolism and didn't have to work that hard to maintain my weight, but when I got up to my late 40s, things felt like they changed almost overnight - now it gets harder and harder every year to try to stay in shape - but that's how life is - Father Time is chasing us all - so that's why we gotta make the best of life each and every day.