The problem is that your question doesn't make sense
Thought it was clear, but I shall endeavor to make it more so....
You are in iPhoto looking at your pictures, all arranged in the typical thumbnail view.
You click it to bring it up to "full" view to see the actual picture. Now lets say the picture was using a "resolution" of 2" x 2", unlike the program Preview which brings up the photo in its origional scale
iPhoto brings it up using the full available view space and magnifies the picture to lets say 5" x 5" thus making the picture over magnified and pixelated.
So the question is, how do you get iPhoto to NOT bring up items in this over magnified view and just use the pictures standard resolution.
I hope that was much clearer