Fire controls are limited to 1.4m km because that's how far light travels in 5 seconds.
There are still advantages to using big lasers, though. In addition to the previously mentioned damage reduction due to range, remember that because of how armor works a 20 point hit is better than 4 5 point hits. An 80cm Gamma laser is probably going to put a hole in nearly anything and damage internal systems, whereas at max range the 30cm would probably only be sandpapering armor.
Alternately, you could use a lower wavelength laser to reduce the max range, and save on cost. An 80cm far Gamma Ray Laser costs 3,888 for 20mkm range, an 80cm visible light laser costs only 648 and hits to 3.36mkm. By comparison, a 30cm Far Gamma Ray Laser costs 1469 for only a range of 2.88mkm, and does a lot less damage (All numbers with capacitor recharge rate 25).