Imagine being 6ft with full armour and swinging a sword at a shield wall you would be gassed in a minute. The romans rotated their troops in the shield wall to keep them fresh also.
Strategy and insurance probably was their main advantage.
Even not counting the lines rotation... on top on that the roman training made stamina the most extremized trait
They had long ass marches with 50kg of equipment on, then you have to build the camp, then the day after you either had to repeat all of that, go to battle or train hard
Then repeat, was a no rest lifestyle
Fight training itself was made to work on stamina (and strenght), they used wood replicas of shield and gladius that were 2-3 times heavier than the real counterpart, to simulate when you're exhausted and get used to keep fight on even when limbs feels heavy af... the result was that in battle the regular weapons felt light to them
I remember somebody of the time pointing out that the clashes with big northern enemies usually made clear the difference between still green young soldiers and vets.
Young ones where the ones that were impressed and scared by these big men first charge, wich often was all-in violent rush with savage war screams and the swinging 100% trying to wreck roman lines and kill as much as possible
Vets were the ones that knew they just had to keep discipline, stay strong against first impact, weather the storm and then enemies will start to get quickly tired and slow down, creating big openings for the roman's quick stab
To make a fist fighting comparision, north warriors approached the melee like a bar brawl, where people throw big from the start trying to gain upper hand and looking for quick KO
Romans approached the melee like professional boxers, that know the fight may last for some/many rounds and you work keeping guard tight to either cause cumulative damage or wait the right moment to land a decisive strike
manlet with a spear (7-9') will make short work of a 6'2 athlete with a sword. now add in formations and strategy and it changes it completely
Roman style of melee was'nt based on spear game though, that was more greek's way
Roman's pilum was designed as a short distance launch weapon, the melee was done with the short gladius and eventually even with the pugio (dagger)... romans tried to force the closest distance possible
To do again fist fighting example, roman was like a stocky short boxer who's strong af in the clinch and try to make a match like that against taller opponent to negate his jabs/cross/long hooks