ferment

I'm doing the best I can to follow Richard Bertinet's sourdough method set out in "Crust." One thing I'm quite confused about is the volume of ferment. I bake about once per week. Bertinet suggests you should have 800g ferment; take out 400g for two loaves, and then refresh with an equal mass of water (i.e., 400g) and twice the mass of white bread flour (i.e., 800g). So after taking out the 400g and feeding the ferment, I have 1200g of mixture. If I kept this pattern up, the ferment would just keep getting larger and larger.