Here is an interesting picture (well it is at least for me lol) of three small gauge swatches each 20 stitches wide and 21 rows tall. The top two were knit on the same size 5 needle, the bottom swatch was knit on a size 3.
Garter stitch fabric has more rows per inch than stocking stitch fabric but what surprised me was just how much wider the garter stitch swatch was than the stocking stitch swatch. You can imagine how this difference would multiply over 200 stitches. To get garter stitch fabric the same width as the stocking stitch fabric I had to go down 2 needle sizes.