Are you cleaning off the bearing shells before fitting? Is it one cap that is going tight? I always fit pistons sequentially, checking the drag on each one as they are fitted. Are you oiling the caps and rods?
Has the crank been reground? Are you checking clearances with plastigauge?
You are going to be loosening them, for sure. You may well see where the bearings were rubbing. Make sure caps have not been swapped between rods.
Last but not least, is your torque wrench accurate (although it would need an excessive over-tightening to distort the rods to that degree. In days of old, caps were tightened onto rods until the bores were round; that was then the torque for that particular rod when it was fitted in the engine.
Edited to add: Did you check ring clearances before fitting the pistons?