Hmmm, my mom pretty much encouraged healthy eating, but was never over-the-top about it. My dad's been overweight all my life - ranging from mildly to a quite large belly. However, we were only allowed a chocolate bar once a week, a Coke-drink once a week and encouraged to drink things like juice & Kool-aid and stuff in very limited quanities. My mom was a SAHM and therefore made most meals rather than pre-packaged foods. Granted, we never shied away from things like chicken fried steak or a good meatloaf, but overall I think she cooked relatively healthy and for a while made a huge effort to help my dad lose weight (and there made everything baked, not friend, low-fat, etc.).
As I got older, she would definitely make a big effort to curb my eating & help me (or tell me) to lose weight, but not in a derogatory way. For instance, she'd always tell me not to eat huge bags potato chips or lumps of cheese & I'd get chastised for doing so. I was generally overweight (but not TOO much - just slightly) as a pre-teen/teenager, but I guess other than encouraging me to get more exercise, there wasn't much she could do about it, I didn't really have any BAD habits - I just always had puppy fat!
Through a concerted effort of exercise and being really careful with what I ate, I finally got down to a great, normal weight in college. I've never been skinny, and know I never will be - as even that last 5 lbs were impossible to shift with 4x-week workouts & close food-watching. Over the past few years, that weight has steadily creeped back on, and then some, mostly through sheer laziness.
Sorry, I digress! Anyway, I think parents MAY be a small factor for some, but obviously it's so many different things that it's impossible to pinpoint.
Edited to add: for what it's worth, my (older) sister has a pretty unhealthy relationship with food/diet. She's ALWAYS on a diet and can get truly obsessive about it. She's obsess about exercise, too. Sure, she looks great, but I know that level of fixation cannot be good - and she's done this in the past, proving that it's not sustainable long-term (seems to be a stop-start, stop-start thing).