The Museum of Natural History had a horse model dressed in full armor. You’d need a ladder just to get up on it, nevermind handling your weapons with the weight of your own armor. Another thank you to the Creator for not letting me be born a knight. :)
I figure the operational size for both animals was pretty much the same ~ because anything beyond human scale would put them beyond "control".
It's only in modern industrial times that we've gotten into using machinery that dwarfs us, and that's only because we build in OFF SWITCHES. Elephants and horses have an off switch but it's buried under feet of flesh and gone. You need a spear to reach it! Last thing you want is an animal you can't turn off ~ like a bull African elephant rampaging in must.