Hm, interesting. The Inexpugnable class is twice the length (and maybe more in volume), but well out of date. Technology of guns and shields advances slowly... but it *does* advance. However, 'how many fighters you can cram into a ship' is more static, so the Inexpugnable has a larger small craft load by a good margin, and even with old fighters that matters a lot.
Despite the size difference, it looks like the ISD actually has more guns.
It's a close enough fight that I think it'd depends on tactics, the ISD should try and bring things into a heavy gunfight as quickly as possible, close and heavy and with fire focused on the important parts of the enemy. The Inexpugnable is probably better off trying to make it a long-range duel, give it's fighters more time to work, and let it's larger size absorb lots of damage to non-vital areas.
So, I'd guess probably pretty close.
If it's an ISD-1. If it's an ISD-2, it'll pound the Inexpungable to scrap.