It has a been a bit of a mixed bag for me.
I semi-retired 3 1/2 years ago going from very full-time work to going into the office once a week (about 6 months ago that changed to not going in at all).
Overall, I've lost about 20 pounds since that time.
However, at one point I had lost about 40 pounds.
The good - in the early days after semi-retirement, I focused a lot on weight and fitness. I went to the gym regularly and had a personal trainer, worked on my diet with WW. I definitely had more time to do these things that I did while I was working.
The bad - When we downsized and were involved in buying our current house I didn't have time to do most of the above as we were looking about 60 miles from where we lived and we did a lot of driving to look at houses and ate out a lot. Then, when we moved the convenient gym wasn't convenient any more and I regained a lot of what I had lost.
The ugly - Retirement does give you more time to exercise and eat in a healthy way. That said, I found that for me (and for my husband) we are much more sedentary. We now have a 1 story house that is not that large. Since I don't go to the office every day (or any day now), I can often go several days without leaving the house. We don't have a need to go out all that often, mostly to the grocery store a couple of times a week. So, the result is that even if I exercise (we have home equipment now), I still find that I spend the vast majority of every day sitting - mostly at the computer. Yes, I get up and do stuff around the house, but that doesn't take that long. I have a Fitbit and find that if I am home all day, I will only take about 1500 steps unless I make a concerted effort to take more (one reason we bought a treadmill). I am exercising more (treadmill, Concept2, weights), but to some extent that is balanced by the fact that I am more sedentary than I was when I was working full-time.