I went through the same surprise with Lee sizing dies (steel, not carbide). I found my Lee dies were under size by around 6-8 ten thousand of an inch so slightly less than a thou. I also found out they size correctly if one uses Lyman #2 (locally made, but pretty close to the original). Using 20 to 1 alloy I found the boolits under size as you did.
Using an expandable cylindrical lap and a grinding compound is one way to go. If you choose to go down this route I suggest no bigger than 10 micron grinding paste (grit no 1000 and more, if you can find 5 micron that's better). It'll take a little longer but you can always remove more, while putting it back on is not so easy.
However, to open my dies I used automotive wet/dry 1000 and 2000 grit sandpaper with some thin mineral oil. I used a lathe to spin the die, but If you haven't one you could use a wooden dowel. Size one end so you can hammer it half way into the die from the top (bottom 40% does all the sizing, upper part is just clearance hole). Chuck the other end in an electric drill and you have a way to spin the die.
Then take a piece of 1500 (1000 if you are very careful) auto paint sanding paper, roll it around a wooden pencil in a direction so die rotation tightens it. Spin the die, put the pencil with sandpaper in and now is the most important part. Do not allow the pencil to remain in one place. Move it in and out while rotating it. If you just press it in one spot you'll open the die unevenly. At the drill set to 1500rpm give it no more than 2~3 seconds at most for the first time. When you pull the pencil the sandpaper should be grey. Clean the die very thoroughly, lubricate and try sizing few bullets of your chosen alloy. Measure and decide if you want to open it some more. Those 2~3 seconds should open it up at least 2~3 tenth thousands of an inch. Ideally you catch it when it is a tenth or two too small, then you use 2000grit for the last bit (also no more than 3 seconds).
Please note doing it only makes sense if you can reliably measure to few tenths. If the best measuring accuracy you can repeatedly get is one thou (nothing uncommon) you may get a nasty surprise in a form of: the die is too small, next time you measure it is too big
All those random movements combined with small grit average out and result in not going out of round if you open no more than up to a thou. If you're opening more a lap is much better.