building muscle is simple... you lift enough weight to cause micro-tears between the muscle and the bone... the scar tissue that grows back is muscle...
Swimming after a weight workout won't cause you to heal those micro-tears without creating scar tissue. It does however lean up your body, causing you to have less fat. You need the fat to maintain the muscle. That's why you don't see any super huge long distance runners or swimmers.
If your body type is a small frame without a lot of fat and you are trying to bulk up, long distance swimming and running might not be the best idea because you will lean up... but if your just doing a few laps at the pool, or better yet, sprints... that shouldn't hurt... it might even aide in the recovery process.
I think I know what the guy was talking about though... when you work out with weights, you can see that you visibly look bigger... that is because you have ripped your muscle and it is swollen with blood and other fluids... giving a bigger appearance, the laps may cool your body and reducing the swelling by aiding in the blood flow through the muscles... if you are looking for that immediate shirt off in front of a girl pose then do it before jumping in the pool
otherwise it won't cause you to lose the muscle you built!