Seriously - what was it that scared Gandalf to look more into Bilbo's ring?

[Mithadan]

I seem to recall some mention from Frodo or Bilbo that the Ring was placed on a chain because it sometimes changed size. I could not find such a reference in Shadow of the Past (which does mention that Gandalf's interest in the Ring shifted from professional curiosity (when Bilbo found it and disclosed the "true" story) to alarm at Bilbo's conduct during the Birthday Party (which prompted Gandalf's visit to the archives in Minas Tirith). This is an interesting observation, Priya.