The portraits created by the ruleset are 72x72 based on the "portrait_char" template using the portrait icon defined by the "charlist" portraitset.
When an icon is placed in a generic control, the image is centered not stretched. Even if it was stretched, it would look grainy, since you are interpolating a 72x72 image to 140x140 image.
You would need to override the portrait base and mask graphics, override the charlist portrait set, and deal with any issues in the character selection window, character sheet or the character list in the upper left tabletop. (There may be none, but those are the places that the portrait_*_charlist icons are used.) Or you could define a new portraitset to use on the PC sheet. (maybe portraitset name="charsheet", and the resulting graphics would be "portrait_*_charsheet".)
Regards,
JPG