You can't beat something with unlimited CR and good flux+shields+PD by attrition. Any attack will do nothing, unless it passes a critical mass of firepower.
When that critical mass of firepower is hard to achieve with 60% of the battle's DP, max battle size factors heavily into victory.
A strong Nexus is virtually unbeatable with only 120DP, though it'd be a pushover with 300DP.
While not exactly a bug, I'm assuming that it's an unintended behaviour to have beating something depend more on something in the options than anything else.
This issue occurs to a lesser extent with Star Fortresses (it's worst with high-tech), but Star Fortresses get a lot of their strength from exterior drones (which don't take lots of firepower to destroy) and are weaker overall, so they can be beaten with 120DP although with difficulty.
Perhaps as vessels approach and surpass 100DP in value, it should be required that they have parts that are not optimally protected by the whole? Star Fortresses are already slightly like this since you can destroy their drones first to weaken them, but a strong Nexus is slightly more strength than a Star Fortress packed into a small well-shielded lump.
Maybe a strong Nexus should have surrounding built-up asteroids that have self-defence weaponry and fighters, and beams that draw away flux from the main hub.