i know this is going to sound horrible, but I ride saddleseat as well, morgan s and saddlbreds, with Anne Speck at Rancho Del Mar in San Diego CA.At our barn (personal experience being a 2 time reserve world champion equitation rider) we try to be as slim as possible.Some girls have eating disorders, other like me, just try to get by. I have a tall frame with a metabolism that is sure as heck slowing down...my butt is big... no doubt about it... i keep my legs not big. The main thing is... your legs arent that much muscle if they get tired. I thought i had muscle ( my excuse for my thighs) until i just rode without stirrups ( sooo much harder than riding bareback) for an entire lesson- we are required to do alot of without stirrup work for patterns.. but not for extreme lengths of time.Riding without stirrups with increase your leg strength and keep up your stamina!!!!!!!!! we do it all the time now, but hte first time was a shockeras for ankles, i know your problem, mine pop out of place sometimes... but as an equitation rider i need to push my heeels down... honestly, just stretch before you ride... it will make your ankle problems dissapear
good luck