Okay, after a long time lurking in this part of the forum, I'm going to add my .02.
First, to the OP:
I think your concern is well founded. But the best, if not the only way to evaluate the situation is to put BF on the horse for a few minutes and observe the horse. Mounting and the first few steps away from the mounting block are crucial. I have seen horses *clearly* indicate at the mounting block that they are struggling to handle weight. Let BF walk around for 15 minutes and watch the horse. Next day examine the horse for tenderness or soreness. If the horse tolerates the 15 minute walk and shows no signs of irration or struggling, I think the walking trail ride on flat ground would be fine.
Now, some details about what informs my opinion -
I am a large rider - 5'7" and between 225 - 245 pounds. With tack, that's 240 - 260.
I have two horses - one, a 15.1 QH with good weight carrying confo. Broad back, short coupled, good shoulder, good hind end. He's about 1000 - 1050 #, so I'm right at 25%.
The other horse is a 14.2 classic foundation QH - he's built like a brick with legs. I refer to him as SEBUQH, Small Emergency Back Up QH. VERY broad back, very sturdy little guy, probably aobut 900 pounds, which puts me at 28%. A little concerning.
BUT - when I was younger and thinner, I was a very experienced, competent professional rider and under the fat, my muscles and bones remember the previous skills. I am very well balanced, I can ride in two point for long distances, and I still have an educated following seat. Not that I could ride a big moving horse in medium or extended trot for long, but my seat stays soft and following in the downward transitions. Would all this be even better if I were thinner? Sure. Am I a less abusive rider than a thinner, unbalanced rider in a badly fitting saddle? Of course!
Here's how I ride my horses - I get on from a very tall mounting block and keep my weight out of the saddle until the horse moves off. I have a good saddle that fits both them and me. I mostly trail ride; if I want to do flat work, I ride in a large open pasture, avoiding small circles, sharp turns and abrupt transition. I warm up in two point and usually only canter and gallop in two point. They're lightly worked; nothing strenuous, no competions. The only things I jump any longer are logs in the path.
My horses are perfectly calm and gentlemanly at the mounting block. (Tension and nervousness at mounting are SIGNS, pay attention!) The are relaxed, forward moving horses who seem to enjoy their work. The bigger horse does this cute thing when we come to an open section of trail where he squeals and roots at the reins, asking if we can please move on now. They are 13 and 12 and have never have an unsound day in the 4 and 5 years of my owning them.
Most significant is that is you look at photos from when they first arrived and now, not only have their toplines not fallen, they have *improved.* I watch their toplines and backs like a hawk, looking for soreness, white hairs, atrophy and heat and haven't found anything yet.
So I'm pretty confident I am not damaging my horses. However, I don't take the 14.2 foundation guy away from home much, and I wouldn't have him as my primary ride. Why? Not because I think I'm hurting him, because I am fairly confident I'm not, but because I do not want to put up with the judgement and comments of others.
Would I put a beginner rider or a bad rider of my size on either them? No, absolutely not, and certainly not on a regular basis. I did teach a friend about my size ONE beginner lesson on SEBUQH, but she mostly trotted in two point and had a few short attempts at learning to post. If she was serious about learning to ride, she'd have to have a more suitable school horse.
Oh, and the furniture, beds and toilets in my house are all in excellent condition.