why does a 7.5l pot and a 30l pot have the same depth?

when the pots get to around 7 litres, the depth of the pots between 7litres and even 30litres only changes by a fraction of a few centimeters.

i was going to use 12litres as my final pots, but whats the point if they are only 1cm deeper than my current 7.5litre?

i know roots grow to the side aswell as down, but within a few days, the roots will show at the bottom again.

any suggestions?

nobody? or was it a dumbass question?