As far as i'm aware, the browsers and Wordpad also use Windows's font lining system, as described here:
http://msdn.microsoft.com/en-us/goglobal/bb688134But as you can see from the screenshots, the other programs render that glyph the same size as the others. Lucida Console indeed does not support that character, which appears the same size as the rest of the text if you use Arial Unicode MS or Verdana. I believe the glyph actually comes from Arial Unicode MS on my system. Therefore, if i switch the font in mIRC to Arial Unicode MS, that glyph appears larger, the size i would expect it to appear.
In any case, i think it'd be nice for mIRC to have it fit the rest of the text, much like Wordpad and the browsers do it. I can only assume this odd sizing is a side effect of the way mIRC renders characters, which is to say it probably doesn't use typical Windows text controls. I know kvirc tries to draw things on the screen manually.