you've inadvertently pressed the CTRL key and moved the mouse wheel a notch in either direction without realizing it?
I don't ever rely on the Tools/Options method and instead always use the fast CTRL/mouse wheel...then it sticks all the time to what I want from site to site.
Sometimes, you'll find that some websites FORCE the font size on you in their coding and there isn't much you can do about that, but if the same site keeps changing on you, I'd use the keyboard/mouse trick to force IE to hold it to what you prefer.
Every now and then, the font size on web pages I'm reading changes. Not while I'm reading but after I close and then go back. It will go back and forth between small and medium.
Win XPpro IE 7