Oh, ok. Cast on stitches are smaller than the knit stitches. Your tension may have relaxed a bit, too.
This is why so many people knit flat on circular needles. You can fit a lot more stitches, and one side doesn't get heavy like straight needles do.
If you want to use the straights, maybe you could knit two thinner panels and sew them together. I'm sure the cat won't mind.