Yes, of course the window has scroll bars, without them it would be impossible to see the whole image.
What is happening here is some confusion over what "full size" really means. I have just spent about 3 hours on the search and I can see that just about everyone who is working with image galleries has the same and similar problems, so I have broken down what I can see into 2 definitions:
#1 - A full size pop-up window that displays an image the maximum size possible within that window WITHOUT scroll bars.
Example: An image 3,000 pixels wide by 1,500 pixels high is NOT really shown full size, it is only shown the maximum size possible dictated by the users screen resolution within the "full-size" browser window. People can not see the real quality of the image because this is a "re-sized" image.
#2 - A full size pop-up image displays the image at real full size and the pop-up window adjusts to the image size WITH scroll bars.
Example - An image 3,000 pixels wide by 1,500 pixels high is huge and is way off screen, however the image can be scrolled and viewed and downloaded. People can see the real quality of the image because it really is displayed its maximum size REGARDLESS of users screen resolution, and they are seeing it "as is".
I use option #2, have done for years, and it needs only 3 lines of php/java code and works with all browsers, all images, and all screen resolutions.
So perhaps to resolve this we would need to clearly identify just what a "full size pop-up window" or "full size pop-up image" really means, and which of the two styles of pop-up the people would prefer to use.
I will happily paste my current code here for solution #2 because it really is so simple - You click on a thumbnail which displays a full size image. Click on the full size image and you return to the thumbnail. Easy! However out in the forums on the internet I have seen heaps of complicated coding that just do not work across all browsers or all resolutions, are unreliable and do not seem to be what people want.
Anyways, FullBest wishes all - and happy headaches

size image or full size window - What do people want?