Not all fibers do that. There are soluble and insoluble fibers and insoluble ones make poop bigger much more than soluble ones.
Metamucil makes my poop bigger and Benefiber actually too runny and not big at all
But - it could be just me

If you eat a lot of fresh mango, pineapple, or oranges and beans of any kind and yams for example - your poop should be very soft and nice ; ))) I had a chicken breast with pineapple yesterday and my poop this morning was gliding out like "a dream" (yes - only person with butt problems can see poop "dreamy" LMAO) - no feeling whatsoever - I was like - OMG - this is niiiiiceeee LMAO
Best of luck - and yes- add SOLUBLE fiber !!!