When they made a new master hub that year they called the small date. I used Photoshop to create the swelling that the older master hub and the changes it went through to become the large date though the years:
This is the same coins image, and I wondered if the '2' could change with the swelling of the master hub. And it did. All the devices moved closer to the rim, the bust enlarged and the motto got thicker, just the same way the large date looks like.