Note that slots differentiate between tabs, If for Example, you have 6 slots in the Large Item Tab, then 12 slots in the Small Item Tab would be equal to the 6 Large item slots. This is because the slots in the tabs are of different size, so 2 small item slots stacked vertically = 1 large item slot by size.

You can see a detailed shot by Vikkz of the previous inventory layout to the left.