The root mass is way bigger on the root pots..
You could plant a 1 gallon plant in a 5 gallon bucket and have the roots go out to the sides and bottom immediately and circle... The roots don't go back to fill in the space unless they are pruned by the air..
so in root pots the roots go out to the sides, hit air, are pruned, and retreat back into the soil... all of that root mass ends up making the plant bigger and stronger..
I have some tomatoes on my deck in roots organic mix and rootpots.. and some not in rootpots.. lemme go take some pics..