There’s very little Jedi in the Star Wars Universe so it’s reasonable to consider the advancement of prosthetics in this universe indipendent from lightsabers.
Funny thing is that limb slicing is actually the primary goal of several lightsaber techniques, since the opponent will usually be trying to guard their vital organs and will leave the limbs relatively more exposed.
Maybe the length of the saber doesn’t scale linearly. Maybe there’s some extremely fast dropoff in effectiveness after a couple of feet.
And the power limitations of the phasers seem to me like they require some kind of capacitor to charge up and release in a burst. Plus maybe the tip of the phaser would overheat under sustained fire.
And sure, you could say “add more tips”, but we’ve tried that with gun barrels IRL for the same reason. They’re heavy & unwieldy so they only get used for mounted guns, like with the minigun.
We have been shown that there is some weight to a lightsaber when lit though. Like in Rebels and the Mandalorian/Book of Boba.
And this weight is different to how a normal sword would work. The length adjustment is to finetune this power/weight distribution to fit the user and their style of fighting. Which is why we get shotos and claymores, as well as different sizes for different sized species.
Actually if I remember correctly, younglings use weaker sabers that can’t cut through as much as normal sabers do. Most early training is done with specifically training sabers like those.
We have walked on other celestial bodies, but we still burn toxic goop that destroys our atmosphere in order to buy chicken nuggets. Logic need not apply.
I accept your premise that parameters of technological advancement need not influence each other, but assuming that they correlate similarly as on Earth, prosthetics and interstellar travel might have moved forward by the same coefficient.
In that case, walking on the Moon is like using a rock for a prosthetic eye.