I looked it up just now and I was wrong - it's 3-5 pounds apparently not half a stone www.butterflybabies.co.uk/1st-trimester.htm
I absolutely agree that if you are happy with yourself then that is a good thing - I suppose I got a sense that some people were worried about regaining their pre-pregnancy shape which seemed to me like people who weren't happy with the changes that pregnancy etc inevitably presents. If you are happy striving to maintain your shape then yes, sure, that happiness must be a good thing for all concerned.
Ultimately though I do think you get dealt the cards you get dealt - there are things you can do to try to influence the outcome but whether you the birth you want or whether you end up with an approximation of the body you had before is to some extent out of your control.
I had no control over the fact that I had a very large multiple pregnancy, that I got SPD and was unable to move for several months. I could, and did, control my eating (because I wanted my babies to be healthy, but it would have been equally valid if I had been thinking about myself too), but I don't think that influenced the outcome in terms of my looks very much.
Of course we are all individuals, but my personal experience is that your physical and emotional identity goes through a tremendous change when you become a mother and my personal opinion is that it's better not to fight it because it's a struggle you will lose.
If you keep your calm by going to the gym then great for your child. Or if you have been lucky enough not to carry too many physical scars from your pregnancy and birth - fantastic. But if not, it's not really something to get hung up about.
The other point was I totally respect the OPs right to ask the question - she's information gathering to help her make a decision about what kind of birth to go for. As part of that you will weigh the pros and cons to mother and child. But ultimately you might not have a choice about what kind of birth to have. Like everything in motherhood, a lot is out of our control. And I'm sure everyone here would put their babies' needs first.