The first Christmas robin I had I used a 4mm hook. I went to make a 2nd one and it wasn’t until I was a few rounds in that I realised I had picked up a 3mm hook. I didn’t see much point in ripping it all out since it was going fine, so I continued on.
I expected it would come out smaller but I was surprised by how much smaller. 1mm of a different in the hook really does make a difference.