The true jackals (black-backed jackals and side-striped jackals) evolved in ecosystems where there is a lot of competition among large predators. Their small size allows them to be very nimble and agile so it’s difficult for the larger animals that can and do prey on them to actually catch them. It’s not really worth it for a lion to expend a lot of energy to prey on something so small so their presence around carcasses is often tolerated or ignored. So they’re small as to not be to be viwed as a threat and very difficult for most large predators to catch without wasting a lot of energy. It allows them to scavenge mostly unbothered, while having the ability to escape at a moment’s notice.
Here’s one of my favorite vidoes to show just how quick black-backed jackals are. They are often seen walking up to adult lions and nipping at them. By the time the lion gets up, the jackal is in the next zipcode 😂