Actually, they're quite accurate. They should
not be increased in size -or if so, only very slightly.
The average medieval warhorse stood aproximately 15hh -that's 60 inches- at the whithers (shoulder). Especially large examples were known, but slightly over 16hh (5'4"-5'5") was the absolute limit. That's
about where the horses in M&B are now.
Our extra-large draft breeds were selectively bred in the late 16th century on through the 18th. The Shire, for instance, which stance around 18hh (6'2") was developed to pull a plow in England in the early 1800s. It's also one of the most kind, docile horses you could ever immagine. It's ALSO one of the most uncomfortable horses to
ride that you could ever immagine, because of the enormous bredth of its barrel (chest) -our legs just AREN'T made to spread that far apart!!!

(ouch!!!)
As an additional concern, think about MOUNTING those horses. You're in armour. You have to get your FOOT into that stirrup. (and no, you did NOT get set on your horse with a crane, that's a myth. Young knights prided themselves on their ability to
leap into the saddle, in full armour, without having to use the stirrups) On a horse that's 15hh, that stirrup will be about 35" (90cm?) off the ground. Easy, right? At
16hh, it's gonna be 39" (1m) off the ground. A little harder, isn't it! Now try stepping on to a shire at 18hh! That stirrup, the place you have to get your FOOT into, will be four feet off the ground!!!
My friends, you would need a LADDER to get on to your big horses.
