Hi, Welcome to Health care magic forum.
As you describe, the surgical removal of the colon may not cause, weight gain, but as the rest of the intestines are regularized due to the surgery, so there may be utilization of more nutrients from the food and weight gain may be possible.
Post surgical care also adds to the weight gain.
You can keep weight under control by minimizing the intake of calories, fats,fries, junk foods, and oily foods.
Increase your activity by practicing, morning walk, yoga, gardening, and do any work in front of you.
Minimize the sleeping time ,practice late to bed, and early to rise, avoid day time sleep.
Wishing for a quick and complete recovery. Thank you.