I saw this cactus outside my neighbor's window. To me, the pot looks impossibly small for this tall cactus.
Yes, that is small! I guess there is no real big issue of over watering with pots like that! I have seen a few cactus plants like that, in small, VERY small almost tiny pots compared to the plant size in a plant nursery here, but the plants still looked big, very green and healthy. I'm sure the cactus (above) would take off if the pot was a little bigger though. I had a very healthy Thanksgiving cactus given to me once that was two and half feet across or more growing in a pot the size of a tea cup and had been for many years! It even flowered every year. I felt a little sorry for it though like that and gave it a little bit bigger home.
These plants are real survivors, despite their growers. One person told me she was very unhappy that her cactus hadn't bloomed this year after being in the same pot for 12 years and all she did was water when she thought about it. That's not even soil in the pot after all this time! I'd give it a bigger pot also.
