It's pretty standard for magic items to change size/shape as necessary to fit anyone who might want to wear them; it makes a fair bit of sense to assume that the Iron Bands are an offensive application of the principles involved. Changing size probably isn't sufficient to get out of them; if your character can turn his entire body to liquid or something it'd be harder for the DM to justify saying that you remain trapped, but ultimately there isn't usually any RAW for such corner cases, so it'd have to be negotiated.