>>1013183His seatpost is too large, not small.
>>1013156That sort of difference won't cause any harm, if you can find a way to do it. Ideally you'd want a reamer but that's going to cost a fair bit of money. I'd look into a sanding flap wheel you can stick in a drill that you can also put on an extension so it'll reach far enough down into the seat tube. You could also DIY something similar with a long rod and taping some sandpaper to it.
