Your legs become stronger and more muscular, but are they thicker or slimmer? Perhaps it dependes on the swimming style as well as different parts of the leg, e.g. thigh versus calf? It will be interesting to hear what real results long time swimmers have got, rather than conjectures.:D
What you are asking is a really hard question because I think it depends on what kind of swimming one is doing, what strokes one focuses on, how much one is eating at the time, and how somebody is naturally built.
I had large quad muscles before I ever started swimming, but did not notice that they got smaller or bigger from swimming. They most definitely got smaller when I started running and biking but this was probably because I was in negative energy balance most of the time.
My arms definitely got bigger from swimming, but legs, not really.
Swimming made my legs bigger, definitely. But I had been running previously. I do a boatload of kicking and use fins though. That really strengthens the legs.
I've always been told I have "bird legs" and I've been swimming since I was about 13. Currently I still swim, I don't power lift, I don't run. However I do spend about four to five miles a day walking my ludicrous dog.
No science... just observation.
I have to agree with Elise! It depends on what you are doing for a workout, how intense the kick set is (or how hard you are kicking, etc). Also, if you are using fins, proper technique, etc....too many factors to really give an accurate answer.
