I disagree that they look like fantasy weapons. I'd also disagree about your classifying the longswords as greatswords. Sword taxonomy is a mish-mash of actual medieval terminology, Victorian interpretation, and modern casual terminology (including from Hollywood and RPGs). However, the 'greatsword' is generally agreed to be a sword to large to be wielded one-handed. This might be due to weight or it might be due to length and balance, but in any case, I would say that the BG3 longswords look about right for 'hand-and-a-half' swords - which are actually longswords in this context (definitions might be different is the game was set in, say an Early Modern/ Renaissance period). Also, how you primarily employed the weapon, and the different techniques used for each type of sword, might alter your definition.
Going by the length of the blade compared to the human figures, the sword seem about five feet long - reaching to the human-sized figure's shoulder if the tip was on the ground. This is a little long but about right for a medieval knight's longsword. As I say, a bit long perhaps (i'd be looking at four feet), but not yet in the greatsword category where the sword was often as tall as the user. Remember that the medieval longsword was not designed primarily as a one-handed weapon; it was a two-handed weapon that could be used one-handed if required (such as whilst mounted). I think generations of AD&D/D&D editions have muddied this point considerably due to frequently pairing it with shields.
As for back-scabbards, however, I am with you 100%. Horrible and with little historical basis. You might sling a sword on your back when journeying, but in any situation where combat was likely (i.e. adventuring), it would be on your hip. The characters would find it very difficult to draw a five-foot sword from their shoulder.
