I agree with Philip, except in one tiny thing.
On the whole the reason for smaller pots is that the medium dries more quickly therefore less chance of rot, just as Philip has said.
My only slight disagreement is that less water in the media means that the plant has less water available to it. On the face of it that seems obvious and correct... until you take into account the human aspect. I find it actually seems to work out the oposite because the medium dries more quickly I water every couple of days or even every day and the plant actually gets far
more water available to it because of the very regular watering than it does in a larger pot where I have to wait a week or maybe two for the medium to fully dry, during which time the medium is holding the water and the roots aren't getting that much of it.
While the roots are getting more water from watering very regularly they are also getting plenty of air because the medium is drying quickly and that combination really helps to get good root growth.
Just my