The cord is smaller than the needles on purpose so the stitches slide easily and you can fit more on than straight needles.
Yes, I know
. I think I get loose stitches because the circular needle is too long (27'') to knit a hat. So it's hard to reach the point of the needle.
By the way, I just started a project (hat). I'm using DPNs and I like it. But, I should give circular needles a chance.