Problem here?

We never managed to work out what causes this. Because it's so rare I stopped worrying about it. I'll return to it sometime, but it's not going to be now as I'm quite busy, so you'll have to suffer it. Sorry about that.
You may already know, and therefore mean you don't know what you can do to stop it happening. If that is the case ignore what I have to say.

This seems to be caused when people quote each other and then accidently remove a [ from the quote tag. When this is quoted it then causes the new posters text to come out small (see the Bull's post with the tiny writing).

Well that's what it looks like anyway...
Great spot and, now that you've mentioned it, I do recall this fix before. The only problem in this case was that 3 other people had then 'quoted' the broken quote and I had to edit all their posts too!