I have not read as much as some on this topic, but i would say that simply having to inject insulin will not make you gain weight. Insulin itself and the fact that its inject can cause an increase in fat cells, but i don't think to the extent that you are experiencing or that some people usually talk about.
I am saying this based on a mix of readings and my own experience. Typically before someone is diagnosed they go through a period of un-natural weight loss, while being able to eat pretty much anything they want (i lost 30lbs+ in a couple months and ate more then ever). This is due to your body burning fat for energy rather then your food intake, so nothing is stored. When you are diagnosed and put on insulin a lot of people expect to be able to continue their eating habits, and put on weight.
Typically the same thing is common when people go on the pump, they blame the pump for causing weight gain, when in reality its because they were likely in mild ketosis and their body burned fat from constant high sugars.
I think the most common (which i did experience) is having to snack to correct low sugars when starting out. I used to never snack, but all of sudden i found myself going low between meals, or having to eat extra because I over bolused. All of this extra food (even like 50 cals a day) starts to add up to a few lbs a month if one is not careful. Its really just poor insulin dosing, or poor carb counting that gets us into this situation.
The insulin itself, is not to blame for lbs of weight gain and fat. That is all based on our eating and exercise habits.