(Note: This was confirmed on a minecraft server)
When I destroyed a fully grown potato in my farm, it only dropped 1 potato! I tried a few times and it still only dropped 1.
How to replicate:
1. Open minecraft
2. Open a world
3. Hoe a grass block and plant a Potato
4. Quick it up with bonemeal
5. When you destroy it, it will only drop 1 potato!