https://www.ufopaedia.org/index.php?title=Item_Weight
plasma weapons are light weight. compare a pistol and a plasma pistol. it's almost half the weight. also, you're not including the weight of a clip. a LOADED heavy plasma weighs 11, the same as a standard rifle. i don't think this is a mistake or an oversight, this is alien technology, made using lightweight alien alloys. i see no problem here.
i mean, heck, just look at the data in this format:
conventional pistol: 8
laser pistol: 7
plasma pistol: 6
standard rifle: 11
laser rifle: 8
plasma rifle: 8
heavy cannon: 24
heavy laser: 18
heavy plasma: 11
Going by these numbers for loaded weapons, a plasma pistol is 25% lighter than a standard pistol, a plasma rifle is 25% lighter than a standard rifle (rounded up)... if a loaded heavy plasma followed this pattern, it would weigh 18 units.
Now I'm not arguing for "realism" here, who knows what an alien weapon would weigh. It just seems curious to me.
If the heavy plasma was intended to weigh 18 without clip, then that would have balanced it out in the early game, when your rookies are still too weak to carry it.
I find heavy plasmas on some of the very first UFOs, and can research them straight away, so why bother with plasma pistols and rifles?
Well, as was said, thanks to openxcom, I can mod this out myself, so I'm good. Just wanted to raise the issue here and hear what all of you thought about it