It's late in the night and I may be a bit thick here, but I don't see how this question can be answered.
When you speak of one inch of rain on the ground, over what area are you talking about? One inch of rain over half a square mile is a considerably less water than one inch of rain over ten square miles.
Because the water volume deposited on the land ( or the sea for that matter) is different in both cases above, the capacity of the cloud and therefore it's dimensions must also differ - it stands to reason that the cloud capable of depositing one inch of rain over ten square miles has a greater water holding capacity.
I await being corrected on this matter which seems quite logicl to me.