Some of the reading I've done for my other problem spoke of keeping fully grown orchids in small pots. Why not give them all the root space they need?
I think it may have something to do with moisture around the roots. Larger pots will stay quite wet (when first watered) for quite a long time, and orchids are not designed to handle wetness around the roots for long periods of time. Some moisture yes, but not wet. Smaller pots will go from wet to damp more quickly, and allow more oxygen to the roots. The white covering (velamen) has a lot to do with this I believe.
Some orchids have more roots outside the pot than in. Chester makes a valid point: especially with epiphytic orchids, air circulation around the roots is vitally important. Yes, if you saw a dracaena with a ton of roots growing up and out of its pot, it would be more than time to repot! But many orchids make their own root space. Some seem to bloom better if confined.--- If your orchid is top-heavy, obviously way too big for its home, or is living in old, compacted medium, by all means repot.
