There are really two issues when it comes to abdominal girth. One is amount of fat. The second is abdominal muscles. If you're fat, your abdomen is going to be large regardless of how good your abdominal muscles are. But if you're not fat, your belly can still sag if your abs aren't tight.
Fat is, in some degree, a balance between calorie intake and how many calories you burn. I've never seen any statistics on which stroke burns the most calories. It isn't easy to measure, because you have the issue of sprinting versus distance swimming. In sprinting, you're burning more calories per minute, but you're normally going short distances with pauses in between where you're not burning many calories. In distance swimming, the rate at which you're burning calories is lower, but you are going to be doing it continuously for significant periods of time. I doubt that statistics exist for distance swimming in any stroke but freestyle.
What I can tell you from my own experience is that the more time you spend per week swimming, whatever you're doing, the more it helps fat reduction. When I became a TI coach, I went through 5 days of training where I was in the pool literally every day, and even though my eating habits weren't as healthy as they would have been at home, I still lost some weight during that week.
I suspect that all 4 of the swimming strokes help to improve abdominal muscles. Probably one of the strokes is best for this, but I've never seen any statistics on which stroke it is. I suspect it would be difficult to measure, because swimmers who know how to do more than one stroke usually want to do a mixture of those strokes during their practices. And if you look for differences in the leanness and abdominal development of swimmers who specialize in a particular stroke, you would be left with the question of whether the differences were the result of the stroke in which they specialized, or whether their choice of stroke was the result of differences in the leanness and abdominal development they already had.