I realize this is an older thread, and I will go check out if you've posted elsewhere since this with updates, lesson progress, etc...(I hope, I hope!!) But, in case you haven't I am also writing this for others to read!
I am a Shorty, big time (5'2") and VERY muscular, though I have EASILY GOT 30#s of fat to lose despite that to be at my very fittest...I currently am at 158-161 and should not (for me) be more than 130#.
Nonetheless I ride weekly and when asking my trainer about this the first day we met, (my concerns being a "heavier rider") & whether she was comfortable with me riding her dressage horses--she has no "schoolhorses", they are her show & pleasure horses, she explained to me that each had been vet checked for her 240#, upper level riding daughter, and all passed...otherwise, she explained, she would not have purchased any of the six.
I am fully aware that riding as a heavier person is a whole different ballgame when you ride WELL and can carry yourself in a balanced, non-jarring manner, vs. being a newbie and flailing all over the place on a horse's back with excess weight. Thus, I KNEW I'd better EITHER GET DARN good and balanced, with a strong core and gentle, soft movement all around OR DROP THE 30# FAST!!
I focused mainly on #1 and whaddya know? #2 is happening at the same time!
At my barn we have a woman who was 400#s and was not nearly fit enough to ride per her own, likely correct assessment . That did NOT STOP HER, though!! What she did was started working the barn....mucking one stall a day, then two, then bathing horses, etc...by the time she had lost 100#s, she was still considered "heavy" @ 300#s, but she was MORE than fit enough cardiovascularly, musculature-wise, and most importantly in her case, she was then MENTALLY READY.
She eventually bought a GORGEOUS, bomb-proof FULL Clydesdale and began taking lessons on this 19hh, 1800# monster! He was what SHE HAD ALWAYS wanted, and they are a phenomenally GORGEOUS PAIR riding along!
She rides dressage, and it takes an exceptional degree of fitness & core strength to post OR SIT his trot and sit his canter-- a horse with his sized stride! Not to mention basic STRENGTH simply to move him forward, leg yield, etc!
I couldn't ride him without a TON of work to adjust to having all that size and power beneath me! Huge kudos to her! She is now even slimmer @ 250#s and her horse completely dwarfs her! (He's SUCH A LOVE and extremely kind and giving as a mount)...if I can one day own more than one horse, my goal is to own an Arab-type cross jumper, whom I can also do light endurance on, AND a draft for an all around pleasure horse & for my husband to ride with me--not because he needs the size (@ only 180#s), but simply because we both really love the drafties and their amazing personalities!...
Sorry to go off on a tangent! Yikes, signing off now and hoping to see more from Sassaflash!