So as a heavier rider looking for a horse with good bone is very important.
correct. Also if your looking to do something very hard on the horse physically (jumping, cross-country, reining, cutting, etc) good bone is an asset.
massive body stands on toothpick legs
disproportionate mass to skeletal structure is always bad. You wouldn't take a typical light boned, small, ladies horse type of mount and throw a 300lb rider on them and expect them to be happy or sound. Those massively muscled, tooth pick legged horses are pretty much in the same position, but carrying 300-500 extra lb of muscle, 24/7. a horse with the bone they have should be 7-800lb, they are actually in the 1200-1500lb range.
The only thing I can come up with that has directly to do with the bone are splint fractures, but even there I'm not actually sure it has anything to do with the horse being light boned. The other thing I can think of is actual bone density, but that would be impossible to see from conformation photos and would require x-rays like for women with osteoporosis...
A light boned horse will have smaller joints and such to match. I think its a bit like building a structure. You can build a very light structure that's strong. if the framing is done correctly, with a light but strong material, and jointed properly, it will be strong(like a well built but light horse) if you then proceed to use all the heaviest materials to roof, cover and drywall, that strong good frame will collapse(like a horse with overly light bones for its structure.) If the building had a solid frame made of thick beams and good joints, and its roofed and covered with sturdy materials, it can withstand anything, but will be heavy. bad joints at any point will eventually bring down the whole house.
Most of the time people are commenting on a horse being 'light boned', its in relation to the rest of its body, not to the actual bone size. what would be light boned in a draft would be very solid bone in a riding horse. The smaller bone and joints mean that the rest of the body should be proportionate, and breeders have made the mistake in recent years of not keeping this in mind when breeding most saddle breeds(qh, paint, appy, Arabian, thoroughbred, etc.)
Too light of bone is like building a pickup truck for hauling heavy loads, but using a frame and shocks from a small car. it will break down. in the case of a horse, the soft tissue(joints, tendons) fail before the actual bone does.