No, they weren’t huge and yes, most knights were useless when unhorsed, among other things because usually the horse would fall on the knight, trapping him.
Knights had to be strong, but not particularly tall or big: after all, the armor was made for them (specially if it was plate), no “one size fits all.” But most knights, at least in Spain, were also farmers. Spending a lot of your peaceful days moving rocks around, chopping wood and hoeing was quite good for keeping a man in good physical shape.
Don Carlos, first Prince of Viana (it’s the title of the Heir of Navarra ever since), spent a lot of time writing a history of Navarra, back in the XV century. He is considered a very modern scholar in that he always indicates who his sources are. He has very little access to Navarrese court chronicles, as the Kings of Navarra saw those as a waste; among his common sources are Parliament records, chronicles from neighboring countries and “what the women say.” At the time, women were the teachers; among other things, they taught History to children, but it was all oral tradition.
About 15 years ago, a group of historians got permission from Institución Príncipe de Viana (yes, named after D. Carlos) to verify something that “the women said” and which modern historians thought those women had been smokin’ something funny; specifically, the stories about Sancho VII The Strong at the Battle of Las Navas.
Some of the items mentioned by D. Carlos had sources other than the women: chronicles from the Aragonese, Castillian and Muslims. The historians had been able to find some of these chronicles, but when they matched “what the women say,” they once more figured that it was exaggeration.
So they exhumed King Sancho and his wife (who had been picked by a delighted ambassador in the court of France, due to her exceptional height) and had a forensics team examine them.
C. 1212 (the time of Las Navas), the average height for the Castillians (according to other archaeological studies) was about 5’4". Sancho was, exactly as the women had said, almost 7’ and his wife was 6’, with neither of them displaying any signs of gigantism. So the modern historians had to admit that hey, maybe the rest of the “tales” about the Navarrese army wearing all-mail because there was no horse strong enough to carry Sancho plus any bits of plate; about Sancho being unhorsed but not trapped and going on a rampage that had the Moors running out of his way; about the Mamluks (soldier slaves, the only people in Spain whose only job was war, at the time) being impressed by his strength and nobility and laying down their weapons… maybe they were also true.
If every knight out there had been two heads taller than the peasantry, Sancho wouldn’t have been called The Strong, you know 
