In the question big-cactus-identification-and-how-long-does-it-take b.nota gives us a nice photo of big cactus. The second thing that struck me was how the soil level is about 0.00000001 inches from the top of the pot. How you water those big boys and is the soil level deliberate so that they cannot be badly watered from the top?
In my opinion, those are more for show than go. Keeping them in pots that size for any length of time would not bring a positive long-term result. If someone dropped those off at my house today I'd take the top 2 inches of soil out and put in some gravel as top dressing. Next I'd plan where to plant them in the ground in my yard. I guarantee that they didn't get this big in those pots.