Panasonic 42.5 - 1.7 lens makes me look fat !

Withing same format, same F-stop value will generate same DOF when subject is framed same size (same magnification).

But, shorter focal length will include more background and it will look sharper, while longer focal length will include less background and it is blurrier.

As you change the camera distance to subject, you change perspective and that changes how roundish or thin does ie. Person face look.

Shorter focal length typically makes face look thinner but nose gets larger and eyes move to edges

Longer focal length typically makes face look rounder but nose gets smaller and eyes get moved to center.

The most flattering perspective is subjectional. As every person face is different, it is required that camera distance and angle is customized to each person.

The focal length does nothing, perspective does.

There is one common thing, about 4.5 meters is the distance that humans recognize and remember the face. So when someone is recalled, the face is memorized from that distance, what makes one of the best if not the best distance.

To get a person look good, camera angle in height needs to be found good and then the person side that is more fitting for theme. A good trick for some subjects is to take two chairs, photographer stands on one and photographs downward on subject, that's about 4-5m from the camera, standing on ground while places other leg on the second chair and leans forward by placing hands on the leg that is on the chair. This will slim cheek and chin, make body look smaller and thinner while head gets tilted little sideways and up. To find a better perspective, chairs distance is changed to closer or person side is changed.

And that doesn't yet include lighting, what is second part from a model pose and camera perspective.

For a m4/3 system a great starting point is 100mm and f/4 for a half-portait or headshots.