This is something that happens a lot in Trek, and irritates me immensely. We get introduced to some new enemy like the the Dominium, the Borg, Species 8472, etc; to start off with they are really powerful, but then suddenly they seem to get a lot weaker, but a lot more numerous. With better technology and a significant size advantage, these ships should easily beat a GCS - but recently we're told that a Dominion ship twice the size of a Galaxy has only three times the firepower.
Anyway, I'm not here to rant! I'm sticking to my "GCS + 30%" strength
for the Dominion battlecruiser for the moment, but might revise it downwards
in future. Pretty much everything else about these ships is speculative
- we've not seen them in action all that much, and we've not seen much
of their interior.