summer sausages contain a lot of fat, but they also balance out with:
extreme high sodium
compressed content, no air pockets
and vacuum sealing.
Also it's expected to be either refrigerated or kept in reasonably cool temp around low 72F
Years ago, a summer sausage fell out of the grocery bag and rolled under the car seat. The following year I discovered it, the vacuumed seal was not broken (that is why it didn't stink) but air pockets formed and it was plump like an American football.
Be mindful of the temperature, the duration before consumption, light exposure, and air pockets, and over salting is necessary. Also, altitude may affect the air pockets expanding.