Here to confirm that it does happen that way.
Example. 6HS per gauss cannon, 6250 tracking (which is the 10% size) at my tech level
1 gun, size = 6.6
2 gun, size = 13.14 (instead of 13.2 less than 4% difference)
3 gun, size = 19.66 (19.8, 7% more efficient than 3 singles)
4 gun, size = 26.16 (26.4, 9.1% better than 4 singles)
Now lets add some armor (2)
1 = 9.44
2 = 17.63 (18.88, 7% diff)
3 = 25.53 (28.32, 11% diff)
4 = 33.26 (37.76, 13% diff)
Coincidentally, it gets even more effective the more armor you go with. Though I've never actually tried out, heavily armored turrets in game and seen their effectiveness. But as an example
Say your ship is a beam ship with 8 armor all around, and you're packing 8 more on your turrets, the difference between 1 gun, and 4 guns is 17.94 per single gun vs, 13.64 per gun in quads, almost 30% Also larger lasers seem to be more efficient in large turrets than smaller ones... but theres rarely a use for turreted large lasers in the game, maybe 15cm at best for mixed PD/AS weaponry.
Also for those of you who haven't noticed before, in the turret design window it says in the text box, how much HS/gun is getting used, by increasingly armed turrets. Its marked as SPW, to the right of the overall turret size.