It's because of the default RAW settings on digital cameras. Most pros will shoot with a full frame digital SLR with at least 21MP (Canon 5D Mark 3 as a prime example). Converting to a standard sized jpg with no compression creates a file with approximate dimensions of 3700 pixels by 5600 pixels at 300 pixels per inch. That means the native size of the image is 12.3" x 18.7". For reference that's about the size of a small poster. If you're familiar with comics, original art is often drawn on 11" x 17" art board. Point is it's BIG. And newer cameras have native resolutions that are even bigger.
99% of the time, the photographer isn't going to change the defaults. And listening to some of the site owners out there, members prefer the original sized images and get bitchy when images are compressed or downsized. Storage is cheap, so there's no incentive to downsize the images. So you get things like a Famegirls set with a zip file of still images at 750MB and a 5 minute video at 250MB.
So it's like going from standard definition to 4K video. All of a sudden, flaws that were hidden before in the small size of the photo are made glaringly obvious in the larger file format. You won't see that zit on her chin in the small jpg that's the size of your hand, but in the native resolution that thing is almost life sized.