Mickey - wondering if you are talking about riding in a dressage or jumping saddle. If you are talking about riding dressage, judging by your DP it is more a lack of core strength and co-ordination that is leading to your swinging leg. You are braced against your stirrups, perched out of the saddle and balancing with the reins. Also when you say that bareback it doesn't happen, leads me to believe that you are really fighting your stirrups.
As mentioned above, you really need to focus on riding with your seat. Sitting deeply, riding every stride and "kneeling" in the saddle to put some bend in your knee and allow the stirrup to naturally support you instead of fightting and bracing against it. Your stronger calves have probably just increased your bracing strength and not created the swing - but increased it to the point where it is noticable.