I apologize in advance for going against the grain on parts of what most have said in this post, but i guess there always has to be a couple people who dont share the same views as the majority (alot of advancements in civilization occured this way :) ).
In my opinion, outside of drug related weight gain, which i completely agree occurs, the rest of the weight gain is mostly based on the same factors any non-Crohn's person has. There was an earlier post where a poster said that research had found that 6% of Crohn's patients were obese. While i think the poster was looking at this as a "thats a high percent" view, in fact, its a pretty good number when you compare with the current numbers in the U.S. that have 64% of americans being overweight, and 24% being obese. Those numbers make Crohn's patients actually look good.
THe same factors that cause the average person to be overweight or obese, i believe are the same factors that make up for, not all, but the majority of weight issues with Crohn's patients.
In my mind these factors are diet, lifestyle, and of course genetics. While some of these factors are influenced by Crohn's, they still are the underlying factors on weight gain.
Diet is affected by Crohns in two ways that i can see, first, i know fellow Crohns suffers, who despite their condition, still indulge in either too many calories, or calories from nutritionally useless foods (pop, chips/snack food, foods high in saturated fats), this in combination with the fact we cant eat a good portion of "good foods" like fruits, veggies, whole wheat, etc, which in fact have Vitamins and Minerals that actually help the body regulate metabolism, and in return body weight, causes a bad combination for gaining weight from bad nutrition.
Lifestyle is also a biggie for both reasons associated with and not associated with Crohn's. For reasons associated with Crohns, many people dont get the proper excercise just because of the condition of their Crohn's, for those of us that can get in the excercise, maintaining weight is pretty easy, but when you look at it from a view of "normal" people, if the average person did nothing but sit around and get no excercise, they would gain just as much weight, if not more because they probably absorb calories better than us. On the side of self induced issues, once again, my two Crohns friends who are overweight have partially caused their own issues. They both have the attitude of "I am probably going to die younger than everyone else, and later in life not be able to enjoy myself, so i might as well have fun now" so they smoke, drink, dont get proper sleep, all of which are a bad combination for both weight gain, and of course the disease itself.
Last thing on the list is genetics, there are the small minority of people who are genetically disposed to be overweight no matter what they do, while this is a small number, it still is a factor. Most of use have the "middle" genetics which allow us to both gain weight easily but lose easily, in an ideal non-Crohns world.
While Crohns definitly plays a factor in all of this, the underlying causes of weight gain are the same for us as anyone else in my opinion, its just we have a lot more roadblocks to overcome to get there since Crohn's prevents us or slows us down from performing all the fundamental pieces of proper weight management.
I believe i am a good example on how different CD can effect someones weight. I have had CD for 4 1/2 years now and been on Prednisone twice, which i on both occasions lost weight, quite the opposite of most people. I then had a 2 1/2 year remission on 250mg/day Imuran. Over the course of those 2 1/2 years i worked out 4-5 times a week, and maintained a fairly strict diet that consisted of about 3500-4000 calories a day, which i never gained an ounce of fat on. I am very lucky in the sense my disease has never gotten too out of control, i have never been hospitalized due to CD complications, and so far my CD has been fairly easy and straight forward to control with Imuran alone. I just entered a flare a month ago, which was mainly stress and lack of sleep induced because of a new project at work, but i am trying to control it through diet and rest, and trying to avoid going on a round of Pred. If you are lucky enough to have CD under control, proper weight maintenance is achievable, like everything it just takes a little extra work, but if you dont have control, it is expected you are going to gain weight just from the above principals alone.