At the beginning "Follis" was an official sealed bag of money that contained a guaranteed amount of money, this was a method to avoid the neccessity of counting large amounts of coins, usually these bags contained 100 equal coins.
So when Diocletian created a new coin that was the equivalent of 25 bronze denarius or 100 asses, it was natural to denominate the new coin as "Follis" like the bag of coins...