Unless the guys using those La Tene III swords were all the Incredible Hulks then I fail to see what difference it makes. They'll still have the exact same physical limitations and considerations as Charlemagne's scara cavalrymen, ranking Viking warriors, Chinese soldiers, Japanese samurai, Medieval European knights, Middle Eastern ghulams and faris or any other sword-toting combatant you care to think of to cope with. Too heavy is too heavy period. If a sword of certain size ends up too heavy using certain materials (although I'm not terribly convinced adding carbon to iron particularly lightens it in the first place, and all-iron and steel swords of the same general design AFAIK kind of tend to have similar amounts of material...) then you make a smaller sword of a sensible and manageable weight.

Now, some specialized mace designs (the Caliphate-era Arab 'amud and some Byzantine and Persian types come to mind) could be impressively heavy indeed, requiring lots of strenght and skill to use and in spite of their cumbersomeness having a habit of pulverizing just about anything they hit (there's an account of a missed 'amud swing that hit the foe's saddle cantle flatly knocking both the man and the saddle off the horse...) - but those are maces. Swords have a bit different overall operating logic, and making them very heavy was never a particularly effective move.

There's also the little detail that it's actually somewhat difficult to put all that much metal into an about meter-long blade without making it too thick to effectively cut with, and then also the heavier the sword the more difficult it is to put real speed into the swing...

This addresses more or less the same issue in detail. Personally I kind of suspect the laws of physics involved were quite the same for the ancient Celts and whoever, but maybe that's just me.