You could remove some (but not all) of the ballance concerns by making this hull a "racial" technology. The more powerful it is, the more racial points it would cost. The downside would be that your enemies might not be able to build these things too, which means you wouldn't get to watch any nice moonship-moonship battles (duelling deathstars!
You might also want to add an additional base hull in this mass class as well, since it seems reasonable to me that if you can build a ship that size you can also build a base that size.
Personally, as much as I'm a fan of balance, I think it's a nice thing to be able to build a few hugely powerful, hugely expensive starships to "lead" fleets with. Imagine using one of these 10Mt hulls as a carrier; you could fit around three hundred fighter bay IIIs, capable of holding over a thousand heavy fighters.