If I've cropped an image, iPhoto sometimes zooms out automatically whenever I view it, so that I can't fully zoom in on it. This also happens in "full screen" view: it doesn't reach full screen at all. I want it to view the whole image. How ?
How much are you cropping the image? iPhoto might be setting the zoom level to 100% (which isn't full screen) so the image doesn't look terrible and pixelated.
The images never look pixillated even when I crop them, although, if you'd use a magnifying glass, you might be able to see the pixels. It's certainly true that that's why iPhoto isn't showing them at full screen.
Also, sometimes, the images aren't cropped to begin with, just imported at low resolution.
Can you check what the behaviour of iPhoto is if you crop down to a really really low resolution? Does it display the image full screen (over 100% zoom) or does it display it at 100% zoom?
My guess is still that iPhoto is setting the zoom level to 100%, whether is looks pixelated or not. So basically, if the photo is too low res it caps it at 100%. There may be a setting to change this but I'm not sure.