Magic
Haha, you can make a single Cell the size of the prison if you wanted to, but of course we have some Quick Room stuff as well, and if you unlock some stuff with your Warden you could make even smaller cells if you wanted to get more prisoners in there.
A number of factors are added together to work out the 'value' of a cell, if the player adds other objects to the cell, such as a bookcase, shower, windows etc, it will increase by one point per object.