Just like damned near everything else in orchid growing, I don’t think there are any absolute rules about potting. However, one must understand what is going on in the pot in order to make it right.
The “big pot” concept comes from two angles - an epiphyte in nature is mostly unconstrained, and the desire to wait a longer time to repot. If that “longer time” lets the media go south, that’s a problem. Likewise, as evaporation occurs primarily from the top and pot wall interface, it can leave the center of the volume soppy and suffocating.
As far as the “unconstrained” logic, a plant wants to be mechanically stable. Loose, unstable media can damage roots and stop them from spreading. A plant in a small pot may get its roots to the pot wall and anchor itself more quickly than one in a big pot. In the wild, the plants can extend their roots broadly to provide decent anchors to prevent them from getting ripped off the host tree by windstorms.
I speculate that aerial roots in phalaenopsis, for example, in addition to being water and nutrient collectors, emanating from higher in the structure, act as “guy wires” to stabilize those big, broad, sail-like leaves that sit on a relatively small base. I once did an experiment in which phalaenopsis growing in semi-hydroponics - uniform, non-degrading mix with similar air/water ratios throughout, courtesy of the wicking ability - were potted in containers as big or bigger than their leaf span. The result? The plants grew fine and the “aerial” roots soon anchored themselves farther out in the container, and they didn’t produce more.
That brings me to another “it ain’t necessarily so” comment about the pitfalls of overpotting.
I used to travel on business around the US and occasionally Canada a lot. I averaged 13 flights a week for 3.5 years at one spell. That was why I worked on media and pot combinations to sustain my plants while I wasn’t there, leading to semi-hydroponic culture. It also afforded me the opportunity to visit a lot of nurseries around the country, and I often brought plants home.
When they were new to the market, I bought a 2” pot of Oncidium Sharry Baby ‘Sweet Frangrance” from Ellenburgers in NY. I brought it home, and as an experiment, stuck it directly into a 24” tall x 16” diameter florist’s cooler bucket I had converted to a semi-hydro pot. It took off and grew and grew and grew for about 3 years before it bloomed with more than a dozen spikes, each loaded with flowers. Later, I tripped in a hole in my back yard (thanks to two Irish Setters), dropped the container, breaking it. I hardly lost a single LECA pellets, as the roots enveloped the entire volume.
