It actually depends on the species of plant you're dealing with. Some plants will only bloom or grow well if they are "snug in their pots", which is about 1 step below being root bound. African violets are an example of such a plant.
Other plants, such as tomatoes, grow roots like crazy. They routinely have huge root systems. Give them a big pot and they'll grow into it within a matter of days. The bigger the pot, the better, as far as they're concerned. Still, if your tomato plants are very small, with only a thread or two of roots, you will be risking them if you put them in very large pots. If the soil stays too wet, you're courting fungal diseases, such as damping off.
Still other plants require very well-drained soil, and in the case of desert cacti and some succulents ... and some palm trees, as well ... soil that dries rapidly. If you surround the roots of these plants with large amounts of soil, it can take too long to dry sufficiently. The wet soil prevents the air needed by the roots of such plants from penetrating between the soil particles. The result of this is root rot, which is very common and kills many a plant. I would feel safe saying that more container-grown plants die from root rot than from any other cause.
Does that help?
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams