Hi Meg,
The smaller potatoes you described will work fine for seed potatoes, if they all come from the more productive plants. The potato itself, typically grow
underground like a dormant terminal bud, with the furthest most growing point from attachment, having a dense cluster eyes organized like a terminal growing tip. So those smaller potatoes will grow well, with plenty of branching, also having plenty of energy to get started, being 1/4 of full size. Just plant the cluster of eyes up, and it will be better centered for seasonal growth. Technically seed potatoes are all the clone equivalent, of the original parent plant; however, sometimes genetic variations randomly occur over the years, so taking seed potatoes from the more productive plants insures your not reseeding from plants that may be infected with disease, while also harnessing any positive changes that may occur on a genetic level, for adaption your region. If when you harvest each potato plant, you can evaluate its overall production, also comparing environmental conditions which may effect optimal production, then weeding out collecting seed potatoes from plants the did poorly, for no explainable reason.
I hope that helps!