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i always wonder what's the explanation for the critical mass of living soil, quess it's has to do something with proper ecosystem but details of it. and yeah bigger is better until it gets too deep and no oxygen is available?
others may have different takes on this. Microbeman is the guy to ask really -
but ive always thought of it as an equilibrium thing. the larger the size of an ecosystem - the more stable it tends to be.
i doubt there is any hard and fast 'cutoff point' - just that as your pot size gets smaller the harder it will be to keep a balanced soil that is going to keep your plant happy up until harvest.
with indoor gardening there are many limiting factors, amount of light, space, ventilation etc etc.
it seems pointless to allow soil volume to be your limiting factor when its very cheep and easy to provide your plant with a decent sized pot to grow in.
VG