I'm going to go out on a limb and give an example of when an ultimatum from my DH, rather than support and discussion, worked on me!
It's not weight-related, it was my MH. I have an anxiety disorder (now properly diagnosed) and used to work myself into real states sometimes. My DH has always been v. supportive and kind, but one day several years ago, I was getting anxious and he turned around and said "I can't take this anymore, Crazy, I just can't" That was all he said - but he meant that if I didn't do something about it, we were done.
I still remember it clearly. It was a huge shock to realise that my strong, supportive husband couldn't live with my problems anymore and I had to do something. So I made a doctor's appointment, got assessed and diagnosed, and have been managing the condition ever since. I still have bad times, but he knows I'm trying (and is supportive) and I'm much better overall.
I hope this doesn't offend anyone, but in my case, telling it to me straight shocked me into action. I don't consider it controlling- he gave me a choice - do something about it, or we're splitting up.