FS2 did involve a whole lot more Shivans, though...I mean, 80 Saths...who knows how many Ravanas and Demons they have just lying around. Maybe they don't even need to conserve their ships, unlike in FS1 where the Shivans did have quite limited numbers (1 Lucifer, 2-4 Demons (depending if you count the multiplayer missions)...
It's very likely that the SD Ravana meant little to none to the FS2 Shivans. I mean, even the first SJ Sathanas they lost is pretty insignificant compared to the 80+ they still had.
FS1 takes place during the closing days of a 14-year long war. Resources are thin, noone is willing to lose even a cruiser. In FS2, the NTF Rebellion has only been going for less than a year. At that point, GTVA Command may be more interested in finishing them quickly or whatever (and the NTF not really give them the liberty of conserving their craft - the NTF just attack with everything they got, and their retreats are really sloppy so its easy to pick off their ships).
SO in FS2, you have both enemies who really don't employ the same strategies as the PVN and FS1 Shivans. In FS1, there was much fewer resources available to put to battle, as a result, major engagements were fewer, and smaller, and sides didn't devote as many ships. In FS2, both the NTF and the Shivans (and to an extent, the renegade Vasudans) don't really have much regard for their own ships, so they just throw them at each other and try to overwhelm each other. The GTVA had to react accordingly, or they would have just gotten flattened by en-masse attacks.