When the Roman Empire conquered a country, the centurions would often conscript locals into the army as a way of recovering from personnel losses before moving on to conquer the next tribe.
Circumstances in the Roman-occupied British Isles were no different. Roman legionnaires would, from time to time, hop over Hadrian’s Wall, capture local men and train them to fight. Now, when you’re picking soldiers, you’d want to take tall, lean, muscular fellows, but after a time, the natives of what is now Scotland ran out of physically imposing conscripts to give.
So one day, a Roman raiding party came back over the wall with a fat kid. In spite of his girth, the legion’s commander was very pleased to have him in the unit.
After all, it’s hard not to get excited about your first round drafted Pict.