Focal size affects per-shot damage
In the cost of the size, and that is what infantry cannot pay.
I mean the smallest laser IIRC is 150 tons and an infantryman in most cases takes 5 tons maybe 6 if you like PWI.
Meanwhile the lowest tech infrared gives infantry weapons that can fire out to 10,000km, which I'm going to assume is not a feasibly usable range for a dude with a laser gun.
For a 12mm focal size infantry "machinegun" compared to 10cm(100mm) focal size space gun if we assume linear progression that results in 1,100 km range, 10% of this is still 110km in case of damage falloff so suffice to say, regardless of what wavelength technology that you have, it won't have any effect on the damage potential of ground weapons. Maybe you could argue artillery - but indirect fire laser artillery doesn't make sense anyways.
On the other hand with focal size you could argue that it also represents the increase in the complexity of laser weapons and not just the raw size that it can be built in. Something which could feasibly benefit a laser armed ground army.
IMO now that I think about it it probably makes more sense to have capacitor tech affect racial weapon tech since it is important for almost every type of energy weapon in the game and allows you more RP flexibility. Only problem is capacitor tech only goes up to 25
but then again rakhas only have 15 racial weapon strength anyways. Capacitors make even more sense in the context of lasers because it usually is the main driving factor when using reduced size lasers - directly attacking the argument that lasers too big for infantry.