okay there from left to right is a size 6 (4mm), size 7 (4.5mm), and size 9 (5.5mm)
i think there is a definite difference but it doesn't really matter if you are making something where the gauge doesn't matter OR if you knit up to the right gauge on the bigger size.
(wow those needles look ginormous like that...lol sorry for the cell phone fuzziness....i am guessing that it is from being too close up. *shrug*)