After reading through all the more recent posts...
Think about it, riding a horse that is 18 hands tall into warfare, do people really understand how big that is? That is SIX FEET at the wither. Six feet tall; every hand is 4 inches, 24 hands? Eight Feet at the wither. Any taller than 17 and the joints simply will not tolerate the kind of strain and extra weight put on them from 300 pounds of armor plus a rider. Not to mention you are just a huge target. massive would equate to lots of broken down nags.
Horses were employed to be ridden due to the advantage of mobility in combat, as well as a height advantage which brings me to another point, you can't be leaning off the side of your horse like a trick rider to try and hit your target, the point of a horse isn't just to charge and look awesome; actually it would be quite stupid to just run people over with your horse. What a death wish! Making horses a little bigger in M&B wouldn't be terrible, but making them a whole lot bigger would be awful. Draft horses are work horses, they aren't meant to shoot all over the place.
Horses in M&B do need to be filled out a little bit and have some conformational tweaks because with their current pasterns they would be a tough ride in RL. But when it comes to warfare and being practical, after a certain point, bigger is not better. I dare say a Quarter Horse would be better bred for war than a giant. Fast, agile, and just the right size with massive bursts of speed you could use to your advantage but not the stamina to keep it up. Arabians are, and I believe have been, wonderful warhorses.
Also, about that comment sitting on the butt of the draft if you have no saddle, not sure where you heard that or if you were joking but that is total BS, she's in the right spot. You have to be a pretty crap rider to damage their spine or wrench their head up so they hollow their back a lot to give them back problems. Swayback
tends to come from two things; having a lot of foals and/or bad riders. A saddle doesn't remedy that much, it just allows the rider a better seat. Being a terrible rider with a saddle ruins their back just like anything else.
Another thing is, warhorses weren't always stallions though they were likely preferred for their vigor and their temper likely did not come from the fact that they were not gelded, not all stallions are evil. Have you seen a war bridle? They are brutal. Most horses likely were not treated terribly kind either. In war a horse is a tool, if it dies you get a new one. I doubt they were perfect, but they had some nasty leverage employing bits back in the day, and they have only gotten worse today. Good thing Horses are inferior to automobiles!
