I have tried several different combinations and can't figure out why the different storage amounts.

30g + 15g = 35 hours
40g =35 hours

60g = 48 hours

How can the factory 45g and 40g equal 35 hours total if my 60g only gets 48 hours.

I tried a 40g + 30g for 70g total and got 57 hours. Why are the upgardes taking so much disk space compared to the factory setup?