I've noticed that on my rMBP, when I try to make a screenshot smaller using Preview.app version 6.0.1 (Tools -> Adjust Size).
When I'm trying to decrease the image size to upload it, the file size often increases.
Compare these two files:
The first one is 3.3MB. Resizing it to 70% of the resolution results in a file size of 2.1MB.
The second one is actually a screenshot of the first image. It's 4.5MB, but again resizing it to 70% results in a file size of 5.2MB. Resizing it to only 99% results in nearly double of the size (8.6MB).
How can this happen? What is going on here?