Q.Why Does A Smaller Pot Causes Bigger Plants
Please explain why a Xmas Catus grows larger if transplanted in a smaller pot. Thanks

Certified GKH Gardening Expert
It is likely that the pot that it was transplanted in was a little too large. The container only needs to be an inch larger on all sides, when transplanting.
Potting up to too large of a container too fast will likely shock the plant. It can lead to infection from overwatering, since the soil remains wetter for longer.
This is the case with most plants, though some will not fit this rule. Still, it is best to use, only, a slightly larger container when transplanting.
This article will offer more information on how to transplant these, successfully:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/repotting-christmas-cactus.htm