The horse com is a rich resource for game designers who want to model horses more realistically/accurately.Here is something useful for gamers from the article found in the title link.
"Her opinions are borne out by historians, who tell us that the mounts of medieval knights were not the towering Shire-type beasties we envision, but stocky, short-ish equines averaging about 15 to 15.2 hands."
In summary the 20% rule of thumb is just a rule of thumb. Its great for most gamers or games when you consider how realistic most games are. Although, going beyond that level of detail helps if our going to start describing the kind of horses that inhabit your game setting or what are the attributes of creatures that perform that role.
The article does point out that until people can breed horses of about 20cm circumference of knee none per 1000lbs of horse this rule of thumb easily hits its limit at 1000lbs.
Again the Square-Cubed law at work. I tried using the horses, bulls, and elephants to plot the curve of a square cubed law. I was hoping I could use it to predict how monsters I made on an earth-like world be like. I would have to look for that spread sheet again.
Reading up on horses, makes me wish I won that $15M lotto last week. I'm sure I can start a eco-tourism business that uses mountain ponies and maybe a chance to repopulate the wild pony population. Maybe use ponies in WWII or Philippine Revolution adventure tourism re-enactments. Sigh...