[Tindomul]

I would think, on first observation, that finer roots are less able to store water and therefore need to work overtime to get enough water to the plant body.
Also, I would think that the more leaves a plant has, the more water it drinks. And pseudobulbs would affect the plant in the opposite direction. Maybe thats why Oncies have big fat pseudobulbs, but then what about Catt types. They got relatively small pseudobulbs, and roots that are thick, but nothing like that of a Phal.
Masdies have thin roots, and no Pseudobulbs. Hmm there goes that theory. But they have much thicker leaves than Oncidiums.

Interesting question.