If it was a straight thread 3/4" tap and it scratched shallow threads in the hole, then yes, the hole would be smaller than 3/4". As I said above, tapping with a 1/2" NPT tap would probably work just fine. Then you could install a 1/2" NPT x 3/8 or 1/4 NPT bushing to connect your filter drain line, or even a 1/2" NPT x compression fitting.
A freeze plug would need a nice smooth hole to seal well, unless you gooped it up with sealant. Even then it might be liable to work loose if you tap a fitting in it. I think that freeze plug holes are cleaned up with a reamer or machined for a smooth finish rather than just drilled with a twist drill.
