I think the Vikings should neither be over overdeveloped or underdeveloped. They used to serve the Romans, had a warrior culture, used guerrila/commando tactics etc.
Vikings... or Norse people of Norway, Sweden and Denmark(+Iceland sorry) grew up being tought to fight. Also they used trell/thralls a lot(Slaves) to fight with them. If the thralls proved their worth they would become freemen, so the motivation was superb. Then we have the old guys who didnt want to die in bed(Sotteseng), which wasnt optimal... They wanted to die in combat to reach Valhalla, so they sniffed some shrooms, dressed like wolfs, howled like wolfes and felt little to no pain because of narcotics(Some speculate if this is where the origins of the werewolf is from). Anyways they were the berserkers, going off with a pension a la warriors death.
Then you have physical activity.. Who here goes to the gym a lot?
Rowing makes a strong upper body, and English scripts describe a taller well buildt people, which was the norsemen.
Even today you can see that, sorry but most scandinavians are taller and in avarage better shape(Personal experience).
So, I think the Vikings knew they were fewer, so they had to preserve manpower at all costs. But they were, one on one on avarage better. Except for knights/men at arms, who were few, but probably equally trained and in shape because of their 24/7 soldier life.
PS: I am talking about the levies vs vikings, the difference in physical shape and training I believe was big.