The tubing cutters make the ID smaller by about 0.1 or more depending upon the tube size to begin with. If you are using VERY small copper tube 1/4" you will REALLY notice the diameter shrink when you use a tubing cutter as opposed to a hack saw.
Even using a hack saw, I tend to ream out the end with a large counter sink (by hand) so that the ID at end of the tube tapers out to the OD, like so...
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