Im honestly not sure they are 'pistols' at all. looking at the actual model the cadian are quite large- not broom-handle mauser size like you would expect but much much bigger. I think las-pistols are actually SMG's- more akin to uzi and mac-11 type weapons. able to be fired one handed but almost as powerful as a rifle but shorter range. the las pistol looks very much like a super compact rifle with what looks like the same power clip. I imagine that also the shooter would get much fewer shots from the pistol than the rifle as there would be more power needed to fire the energy bolt through the shorter barrel. the shorter barrel would also reducethe range
the other reason why the laspistol is so big is that a real scaled pistol would be almost invisible and would have no detail- at least for mere human painters.
however it could be caused by the fact GW think "correct scale" is a dirty word
the mauser looks very much like the cadian las pistol.
the uzi- scalewise more of a match and also the same kind of performance
catachan and krieg laspistols are much more compact....
"Laspistol" isn't, of course, one weapon. It describes simply a compact, one-handed weapon based on the laser technology of the Imperium (if you check out the Tau of PO, then you'll see my personal take on Imperial laser weaponry).
So, yes, there are weapons as large as a Mauser or Uzi, but also as small as a Walther PPK. Digital laspistols are as small as a finger ring, yet just as powerful.
Don't take the tabletop weapons in the 28mm range as a true guide. They are deliberately larger than life to meet the needs of the tabletop gamer (the ability to identify quickly and at a distance what weapons any given model is holding).
By way of example, this old 54mm model is wielding a 28mm revolver and it looks perfectly in-scale: