Size-influencing parameters? (Death Star comparison)

[BRZLWRST]

I've tried to find related topics, but since i do not have this problem on other geometries there might be no direct glue to help.

 

By two different bottom-up approaches i tried to build a mothership in sphereic shape. While the first (half sphere) had a volume of 48 bill m³, the second sphere (2 half spheres) was capped at 8 bill m³. What is it capping my beautiful sphere while other shapes dont get affected of scaling up? I tried many variations of geometries and added/removed crew/mass, to check if it might help. It is possible to scale up the circular floor area including the walls, but since there is no option to keep proportions a beatiful sphere will get lost.

Thx in advance