I am having the same issue.
I’m optimising the images via Photoshop so I have set the WordPress jpeg quality to 100% via the jpeg_quality filter:
add_filter( 'jpeg_quality', create_function( '', 'return 100;' ) );
My original file is 607KB from photoshop @ 2400 x 3106px
All the sizes I resize to are smaller than the original.
1st resized image: 3.1MB @ 2048 x 2650px (how the hell did this happen?)
2nd resized image: 1.5MB @ 2048 x 1280px
3rd resized image: 1.5MB @ 2000 x 1250px
4th resized image: 2.6MB @ 1920 x 2400px
and so it goes on.
It’s only once the resized image widths fall below 1000px that the file sizes are below the original.
Can anyone explain why?
WordPress issue or GD issue?