The size of stitch is determined by the size needle its KNIT onto, not the size its knit off of.
so when you slip stitches for a cable onto a cable needle and then knit off that smaller diameter needle--it doesn't matter.
what matter is what size needle is use to make the stitches.
(remember this (if) when you frog something.. you can pick up the stitches up (after frogging) with any size needle (and a smaller needle is often easier to use!) just be sure to knit them onto the proper sized needle!