What's this manga with a woman with body image issues?
I think you're probably looking for Shibou to Iu Na no Fuku o Kite ("In the Clothes Called Fat"). This manga has a protagonist who is overweight and can't manage to control her weight at the beginning. The scenes you describe of her coworkers and her boyfriend happen in this. Her boyfriend's name is also Saito, matching your description. In order to try to keep from breaking up with her boyfriend of 8 years, she develops an eating disorder, which leads to further complications. The main focus is her psychology and self-esteem issues, as well as the issues facing some of the other characters.

The manga was published in 1997, and spans 15 chapters total (collected in 1 volume). The English version was published very recently by Vertical.