Differing size pipes make much difference?

i like to my own exhausts and found i like to stay the same size out of the headers with either an X or H pipe and reduce it after that. i just did 3 1/2 x pipe on my dart and reduced it to 3" before the mufflers, and i did 3" x on my duster and reduced it to 2 1/2 right before the mufflers, both with tailpipes to the bumper.