Weren't knights ridiculously susceptible heavier weapons that could knock them around though? I imagine a decently heavy sword would be pretty effective at taking down someone is heavy clunky armor if the guy using it was pretty nimble.
I remember reading a book years ago that pretty much said that after a while most militaries were able to craft weapons that were especially designed to take advantage of the flaws in a knights armor. Like the estoc which was a rapier designed for stabbing into the openings behind the joints in heavy armor to inflict damage that made it extremely difficult for the knight to be able to move around or attack due to the already solid weight of their plate armor. And the halberd was specifically designed with a hook on its back so that infantry men could grab knights riding by on horses and pull them to the ground. It's kinda neat reading about the ideas people came up with to take down one of the biggest nuisances of medieval warfare.
They were susceptible in the sense that getting smashed with a pollaxe would knock anyone over from the blunt impact, but the impact would be far more devastating to someone not wearing armour. A helmet would often make the difference between being knocked out and having your skull caved in.
The heaviest swords were the huge 2 hand swords, coming it at just over 3kg, but the mass of even those was concentrated more towards the hilt for balance reasons. The only way to deliver a massive blunt impact was to give a murder stroke with the hilt, holding the blade. Swords were more usually half-sworded and used for stabbing into gaps in armour. Maces, war hammers, and pollaxes concentrated mass more towards the striking end for a bigger blunt impact, but they were slower and more exhausting to strike with. You can't "nimbly" swing something heavy enough to knock somebody flat, so even these bludgeons tended not to be that heavy compared to, say, a sledgehammer. A modern ball-peen hammer is about the same weight as many historical war hammers.
A full plate harness weighed about 30kg, the same as a modern soldier's gear. Knights were slightly slower than unarmoured people, but they weren't sluggish.
Armour got more and more common for everyone to wear, including common soldiers, until muskets became common. 15th century English archers and spearmen wore helmets, maille shirts, brigandines, and sometimes arm and leg armour as well. Most militaries were able to craft weapons which could easily beat armour, and they were called guns. Armour protected against pre-gunpowder weapons very, very well. Marshal Boucicaut, commander of the defeated French at Agincourt, survived barrages of arrows and was taken prisoner. This would have been rather unlikely without armour.
It's important to move away from the fantasy image of knights in armour so heavy they could hardly move slowly heaving around gigantic, overweight weapons. Games like Dark Souls show movements far more sluggish and cumbersome than those in real combat. Keeping weight down was quite a high priority in history, and there was a practical cap on how heavy weapons and armour could be, which was about the same as a modern soldier's gear. This isn't a coincidence, it's the maximum weight a combat-effective soldier can carry.
Higher mobility and better visibility and situational awareness, I suppose.
In a one-on-one fight I imagine a less-armored foe could be quite a nuisance for a fully-armored knight, particularly if he's in a full helm. Long as you took advantage of his limited vision and attacked him from the sides or below where he couldn't see your weapon coming as easily, tripped him up, etc.
The problem is that you have to hit him in specific places, whereas he can hit you in most places. He can protect specific weaknesses a lot easier than you can protect most of your body. A Viking nobleman would have been reasonably protected with a helmet, shield, and maille shirt, but his lower legs would be open to cuts where a plate armoured knight's legs wouldn't. The Viking would have to worry about leg cuts, while the knight could get cut on the legs without being harmed at all. Common Vikings usually just had shields and spears.
