Underpotting orchids

[ArronOB]

I have read many times that some orchids (especially oncidium types) like to be underpotted, in the sense of being restricted to a small pot.

My question is: Is it simply the case that their preferred conditions of water and air movement are more likely to be found in a tight pot, or is there some other mechanism at play. Hormones or something?

Cheers
Arron