What defines Zone/SHQ item storage capacity?

[ydmatrix2]

Without having built any special buildings to increase storage capacity, each zone has it's own default and different storage capacities for different item types, part of which is contributed to the SHQ max storage capacity.

These values are different for the different cities. How are they generated?

Based on city size? Population? Existing buildings? What part is contributed to the SHQ storage capacity?

[Clux]

Im not sure either, but as far as it looks like, the size of the city impacts the maximum stock.