Cacti swell up when they're watered a lot - this is part of being stem succulents. They shrink down when they're using up their water. This shouldn't happen over short periods of time, however.

This depends on what you mean by shrinking, of course. Some of the unhappy times cacti of mine have rotten, they seem to shrink down first.

By the way, Euphorbias are not cacti. botanically.