Gary,
I'm also using Safari on a Mac, but I'm not seeing the issue you have.
After testing with the same popup (Noel's) I saw somthing that I hadn't realized before.
It appears that Safari will always attempt to center the image depending on the horizontal width of the browser window.
So if I have a wide brower window taking up most of my screen real estate, the popup will be centered and large.
But if I make a narrow horizontal width browser window, it will still center it but the popup is much smaller.
Of course, this is dependent on the original size of the uploaded image, i.e. the original poster's image popup shows as the same size as the original image on the page.
People who know what they are doing (like Noel) upload a more generous size so that clicking on it gives a larger more detailed image which is the whole point of popups to begin with.
Have you have this problem in the past? Does it appear if you use a different browser?
What happens if you "Empty Cache" in Safari, or the more extensive "Reset Safari" (under the Safari menu)
