I'll chime in on this one.
I agree with all the reasoning mentioned so far. But you have to remember that a little plants roots don't take up the whole pot. So when you water, the plant 'drinks' from right under the plant where the roots are and the other soil just stays saturated and isn't absorbed into the plant. So you end up with a trade off: A) You water whenever the plant needs water, in which case the area where there are no roots will just stay wet causing problems like mold etc., OR B) The plant doesn't get as much water as it could use because the soil where there are no roots hasn't dried out.
The plant will pretty quickly stretch its roots out over the pot to mitigate the difference. But then you have a new problem. The new problem is that the roots aren't working efficiently. So you water the small plant in a big pot and it takes longer for the plant to use the water, because it takes more water to saturate a big pot. So the soil is damp longer which leads to mold etc. etc.
Growing outside naturally, the plants roots stretch out as far as they can so that they can get maximum water per rainfall. Basically its needs to do it to survive. But inside, in a controlled environment, it's much better to have an efficient root structure since the plant gets watered regularly.
You know what i mean?