As you say they fit ok once you have them on, sounds like your cast on may be too tight. I cast on over 2 needles and then knit as normal. I also have extra stitches at the top of the leg part and decrease as I go down, because legs are generally wider at the calf than the ankle. You can add as many stiches as you think you'll need, I have 12 more and decrease 6 stitches 1/3 way down then decrease the last 6 stitches 2/3 way down to heel shaping. I start with 72 stitches and end up with 60, 2.25mm needles, 4ply sock yarn.