The cast on edge is tight before knitting the first row anyway. It's not uncommon for it to be a bit of a stretch to get those stitches all the way around. But after a few rows, everything fits comfortably.
However, if the gap between the start and end is bigger than a half inch, even with stretching the stitches as much as you can, it's just not going to fit enough to knit the first round.
You could cast on using a larger needle size, that will give you more room to stretch them around, or you may have to use DPNs for the first couple rounds. I'd try a larger needle first, but remember to switch back to the smaller size before knitting round 1.