Why lots of marble potatoes....no big ones?
Certified GKH Gardening Expert
Hard soil or heavy clay soil can cause potatoes to have difficulty growing larger. This is especially true if there is not enough rainfall or irrigation so the soil stays dry and hard. Adding more organic material (compost) to the soil can help. Also, some potato varieties need a long time to reach full size, so depending on when you planted them, they may just need another few weeks to grow (as long as the foliage is still alive).