that it's a jpeg and that when you cropped the photo and saved it it defaulted to a lower compresssion ratio than the original...
say the original was saved at 60% compression, when you saved the cropped version it was saved at 1% compression.
It's just a shot in the dark, and I'm not certain myself that my reasoning makes sense... but it might be worth trying to do it again chcking the compression ratio...
It's usually somewhere within the "options" that appears when saving a jpeg.
Apologies if I'm barking up the wrong tree!!