I always though they must be getting that big from eating something in the mill that I could not see. The thing that I hated the most is when I would turn around and see one of them in amongst the gears while I was also there greasing gear teeth and bearings. I knew I could not move fast enough to get out of there without getting bit. Mice tend to over reproduce while a rat controls its population better to suite the food supply. In other words a rat will eat its own kind when food supplies grow short.
Mice periodically have more offspring than what their local food supply can feed.
This can leave behind a great amount of mice carcasses when the food runs out. Some of the saddest mills that I have seen for sale is when the rodents and insects run out grain supplies that remained in the mill when it shut down and go to eating the fabric of the building and machinery. When I was growing up we always knew that it would be a bad winter when the field mice would start coming into the house. The mouse is the most abundant and fastest breeding of all of the rodents. In ten months, a breeding pair of field mice will have produced three generations of 2,557 offspring.