3/4" pvc pipe seems to have swollen/expanded

I put in a new irrigation system last spring. I hadn't hooked up the drip system last year and am attempting to do so now except the pipe that I put in last year is just slightly bigger that the fitting that I just bought. The extra pipe in the garage (same pvc pipe) still fits the new couplers. I have tried sanding but it did not help. Any suggestions? Why did the pipe swell?

Although I have never run into a swolen pipe I have had deformed ends that will not fit a new fitting. Do you have room to cut the pipe back 1 or 2 inches, that should get you a clean fit.

Is this flex or rigid PVC? Flex PVC has a much lower pressure rating and so exposure to high pressure at warm temperatures could account for the swelling.