Thanks guys, I'm embarrassed to say I hadn't thought about that. I have my multi selector center button set to thumbnail off/on and I must have unknowingly pushed the selector down while pushing the center button. When you turn the display off and turn it on again it shows the display that was on the screen when you turned it off. Then when I went into the menu to turn off the RGB histogram display (I did remember that), it went back to the full size image display.
I guess this is one of the consequences of having big thumbs.