I am going off of practical experience - I find that my heavier ships with the big guns to more damage to ground forces than my destroyers with the smaller guns.
I'd need a lot more details to make any assessment of the significance of that.
Also ground armor is not the same as space armour. High armor simple reduces the chance of damage happening doesn't outright negate it, so although beam weapons can be resisted, bigger guns tend to kill with higher probability. Given how atrocious orbital accuracy is, I find it more important to make the few hits you have count as opposed to spray and pray.
That's poor mathematics. However good or bad your hit rate is, twice as many shots yields twice as many hits.
My bombardment weapon of choice is the 50cm railgun, it is in essence a compromise between volume of fire and orbital anti-armour capability and so far it has worked well.
As for anti-infantry my fleets point defence gauss weapons actually do quite well.
Do you know what tech level and troop quality your targets are at?
My new-model power-armor infantry have armor 22.5, which means a the 10 AP of a 1-point space weapon hit yields less than 20% odds of actually killing them. (If the armor doesn't stop it it
will kill them, since they have less than 20 HP.) Heavy power armor infantry, which I didn't want to foot the bill for, would take that down to 1-in-9. Even light infantry have better than even odds of survival. Well-developed Gauss still probably dominates lasers (not sure about railguns) for clearing light infantry, but against the crunchier types it probably loses out.
Tangentially, orbital fire support bombardment is a definite use case for miniaturized lasers, since the exigencies of ground combat rounds should make everything's effective RoF be 3 hours...