Um.. I mean... the models are kinda long to not have on the back, at least for the longsword and greatsword, especially. The longswords in this game look more like hand-and-a-halfers so they are longer, and the last thing I would want to look at is the end of my weapon glitching into the dirt.
A longsword IS an half-and-a-half sword in 5e (and in most historical interpretations of the term in real life). The longsword was designed to be a two-handed weapon that could be used one-handed if required, such as whilst on horseback. A 'bastard sword' was actually shorter than a longsword in real life, being somewhere between a longsword and an arming sword. The arming sword was the one-handed weapon carried by foot troops, and is probably what the old AD&D broadsword or longsword were closer to.
A longsword was around 4 foot long (obviously designed varied somewhat), so the BG3 ones might be a little bit on the long side, but only by inches.
