From what I've read of comments in several forums, weight-gain after a colectomy actually seems to be a "thing" for a significant percentage of colectomy patients. It is not (yet) recognized as a discrete medical issue by the GI profession, as far as my research indicates, although aspects of it are discussed here and there in scattered medical literature.
Here are a couple things I've gleaned regarding colectomy and weight gain:
The colon produces most of the seratonin in the body. When it is gone, seratonin levels plummet. This can lead to an ongoing low level of depression (or high level, if one has been struggling with depression pre-colectomy). And when people are depressed, they often eat more, hence contributing to weight gain.
Feeling full - or not:
Leptin is the hormone that tells you when you feel full. Eat to satiety, get a surge of leptin, feel satisfied, stop eating. That's the normal cycle. In persons with a total colectomy, leptin is overproduced in the mesyntery and bowel wall. This means you probably have higher levels of leptin in your system than normal. The result is like insulin resistance, only in this case, leptin resistance: your body doesn't recognize that post-meal leptin surge as anything significant, and you are less likely than a normal person to feel full after eating, more likely to snack frequently or late at night, and so on.
I'm still researching this, but it is like putting together a jigsaw puzzle. More pieces of the picture are out there but I have yet to find them. In any case, these two facts alone (mood and not feeling full after eating) can be huge contributors to why one puts on weight.
I also am dealing with significant weigh gain post-colectomy (I also have an ileostomy), and one thing I've noticed that helps is if I take care to eat food combinations that make me feel full. For instance, beans and rice together form a complex carbohydrate that is more filling than either item alone, takes longer to digest, and includes fiber (from the beans) that further gives the body a lot to chew on (so to speak). This helps address the mis-read leptin issue, I think, because I end up feeling more full (like clubbing my leptin-meter over the head with a stick to get its attention) - and hence it is easier to do portion control and eat less.
Aside from this, I have no other advice to offer at the moment except that old tried and true "eat less, move more." But I think it is interesting (and bothersome) that a total colectomy can complicate this picture because of the many ways the absence of a colon throws our systems out of whack.
Good luck with managing your weight! It shouldn't be this hard, but maybe the medical things I've discovered (above) can help you (and others) also get a better handle on this problem.