I think one of the interesting mathematical things about fat ships in this game is that it's not just points denial that makes them good, it's the ability added by the upgrade. I this game, most upgrades are not simple additions, they're multipliers. Engine Upgrade is the biggest example. A Z95 would never take EU because it's not worth 4 points to make a 2-attack PS2 ship better. 4 points to make a 55 point turret better, on the other hand, is trivial. PS works the same way. Bidding up on a named YT or Decimator is cheap PS, because the PS is enhancing the offense, defense, and positional ability of a 50-60 point ship. Everything sort of feeds off each other. Better PS lets you see more opposing ships before you move, which lets you use maneuverability to bring guns to bear, which also benefit from high PS because it can take targets off the board before they shoot, which improves your survivability, which is also improved by maneuverability, which is improved by high PS. If you had a 40 point ship and 2 25 point ships, you'd almost always be best served by putting your remaining 10 points in upgrades on the 40 pointer, because those upgrades are all feeding off of each other.
I don't know how to stop this, or that it should be stopped, but it's not just point fortressing. Fattening up ships makes for a genuinely more powerful list.