Illustrator losing image quality after pathfinder application. am I doing something wrong?

[belvir]

Hi All
Im trying to get some images for screen printing and coming across a problem.
The same image below after merging the black and white on the right side and removing the black paint
in theory I should have the same image left but im left with thinner line... losing all the detail.

you can see the lines are not the same thickness when looking at the images.
I thought that if I merge/divide I should be left with the same proportions.

zooming in it looks really good and the same.