I think the shorter stirrups might be why your body looks disproportionately longer; I was actually thinking if you were in a dressage saddle with dressage-length stirrups, it wouldn’t have been as noticeable.
I only remarked upon it, because I rode at a barn that really shortened my stirrup length and it threw me off balance, throwing me forward, because my upper body is much longer than my stubby legs. The photos of me from that period look so odd to me now, even on horses that are a perfect size for me! I look like I can barely get my legs beyond the saddle, yet my upper body towers over the horse.
When I switched to dressage, I felt much more secure with the longer stirrup length, and even though the horse I currently ride (I don’t own) doesn’t have a dressage saddle, I ride and feel much more comfortable with a longer length. Not dressage length in a hunt seat saddle, obviously, but longer than I used to, and it makes me feel more secure and effective, and better able to use my seat and different parts of my leg.
Obviously, we’re very different body types, though, and I’m actually rather hyper-mobile in my joints (thought God knows I have a million other physical problems with my riding). I’m also not a riding instructor, so do what you think is safe and works for you.