Tuber characteristics are genetic. You can make them them a bit bigger by cultural things like fertilizer but most of it is genetics. One factor is the amount of time in the ground during the favorable growing season. They do get bigger in a longer growing season. and one way to extend the growing season is to start them in gallon pots 30-40 days before your planting date. Remember too that one trick of the trade is to plant a first year "too small" clump whole the next year and the tubers will be bigger the second year. I remember first reading about this trick when a person could not get tubers from Camano Cloud and the second year clump had nice tubers. The trick has been enhanced by the practice of harvesting the top couple of tubers from a clump and then planting the bottom portion of the clump where the tubers are smaller or harder to divide, whole and continuing the practice ad infinitum. A grower said this worked well for a shy tuber producer. All of these methods to increase tuber production are because the breeder did not breed good tubers into his/her product. But when a super nice flower appears in the seedlings and it is a shy tuber maker/keeper and you know that there are ways to grow it , should you toss it or keep it? Most of us keep it, 
Gregor's Mad is one of the names of this one that is a crappy tuber maker but is still gown 20 years later. It is a Danish dahlia and as of few year sago, the breeder(Gregersdahl) was was still alive and in his 90s. It is related to Duet introduced in 1955, Duet is one of the more popular varieties sold in Dutch tuber packs. If you can get some good stock of Duet, it worth growing. Get stock from someone who has grown it disease free in their garden.
