*I cant speak for the mac, but on a PC I can tell you this...*
*What I found out is that if you add a photo on the phone it goes big screen, but if I add a picture via outlook on my pc it looks just like the ad... It may be the phone, it may just be a resolution issue (maybe outlook shrinks the photo down). it could be how outlook loads the infomation, add a photo into your Mac address book then resync and post back if that solved the issue*
It is the same for MAC. If your picture pre-exists in your address book, it will show as a thumbnail. If it's a picture you take with the iphone and choose it for a contact or a picture from an existing picture library you sync to the iphone from your computer, it will show full screen.
P.S. Is it me or does the thumbnail show up on the top left hand corner as oppose to the right?