I have become a regular iPhoto user ever since Elise was born. One day while flipping through some pictures in iPhoto, I got a little frustrated with the picture zooming via the slider. It has no mention of increments as you can see here:
No percentages, no indicator of the zoom factor, nothing! Except for those super tiny picture frame icons on either side of the slider. Anyways, in my frustration, I tried clicking on the icon to the right. Lo and behold, it instantly zoomed the picture all the way up to fill the frame. The other icon scaled it back down to the thumbnail size. Pretty neat!
Just for kicks I checked the iPhoto Help contents and the icons are not mentioned as functional buttons.
If you press 1 or 2 while in browsing mode in iPhoto, that will also scale up/down the selected picture like the tiny icons do.