I'd like to cut in:
Spinal mounts are specific to "making a weapon about 25% and 50% in cm larger than your current highest tech, but you only get one of them per ship". Functionally, a spinal weapon becomes obsolete once beam size technology catches up enough, as large enough beam weapons are identical save for imprecision (only spinal mounts give 19 and 18 cm lasers, for instance). Just as well, the power costs are the same too: 1 power per 1 point of damage, while capacitor tech grows slowly, so smaller lasers in general will have higher DPS per piece than larger ones until you exceed their power supply significantly. And it's worth noting that capacitor tech maxes out at only 25 points per 5 second turn per weapon only at the very end game tech. This means railguns will have a unique DPS lead versus laser platforms
Another thing to consider, also, is that for rate of fire, it is not relevant until the first shots of a conflict have been fired off, which may be crucial for taking out especially large, heavily armored or shielded ships. Which leads on to my next point: lasers do more alpha strike damage per ton the larger they are. For comparison:
-10 cm laser, 3 HS, damage output 3.
-20 cm laser, 6 HS, damage output 10
30 cm laser, 9 HS, damage output 24
and so forth.
Considering that lasers also can have their sized reduced in half by sacrifice recharge time almost entirely, you can get seriously large alpha strikes in small spaces, but in general, lasers seem to benefit most in that their respective technology is incredibly easy to use for multiple purposes: cracking past heavy defenses, ranged brawling, or close in range splattering and anti-missile. Mesons, particle beams, and railguns/gauss can slightly outperform in each of these specific roles, but the fact that a laser can be mounted to perform a majority of these roles well at combat-time with no retrofitting makes them very strong.
On the mention of turrets, i'd like to picture them more as "tonnage you save on engine size, and fuel you save on engine overclocking", as the existence of turrets means you do not need to have all your anti-missile vessels fly like torchships to function defensively.