"It is my responsibility to see to it that they can handle adult women who have developed internet bullying to an art form just to pull in a little money and get a few minutes in the limelight," Josephine wrote in the Resumé daily.
Zytomierska is one of Sweden's most high profile bloggers and her criticism of the advert by gym chain SATS, and the appearance of its subject, prompted a heated debate in the Swedish media last week.
In her post, Zytomierska questioned why it is "okay for women to be plump in Sweden", calling the SATS campaign "disgusting feminist propaganda".
"I look at the SATS advert and am completely flabbergasted," she wrote.
"Josephine thinks it is appropriate to stand completely without make up with lank hair and clearly sweaty clothes in an advertising poster for SATS and thereby shows that she is happy the way she is."
The blogger argued that in other cities she has visited "the women are thin and that is something positive", while recognising that being "too thin" and "starving oneself" is negative.
She furthermore questioned overweight women who diet to lose 30 kilogrammes and then stop when they weigh 70 "just because they look like a person and not like a beast".
The blog post has received over 2,000 comments to date, prompted a response from the SATS CEO, and generated extensive debate in media and social media circles.
SATS pointed that the whole purpose of the advert was to underline that the company welcomes everyone, of all shapes and sizes, into its premises.
"Katrin's blog post is regrettable. It was not just a personal attack on one of our members, it also, in our opinion, disseminated unhealthy ideals," SATS CEO Farhad Jabbari said in a statement.
After several high profile sponsors withdrew their cooperation with Finest.se, Zytomierska returned to the subject on her blog to defend herself.
"The debate over my statement is simplistic and stupid. One should have the right to comment on people's bodies and health in the same way as you have the right to slam people's clothes and appearance," she wrote.
"One should have the right to call a fatso a fatso, if she is a fatso."