When you pulled them up, were there lots of very small potatoes clinging to the roots or was what you see what you got?
From two 18' rows of red potatoes, one plant every foot, I got over 17 pounds of potatoes. That's discarding the ones I poked with the pitchfork digging them out, and the others that a varmint nibbled on. I took about 4 pounds of new potatoes off those same rows as they grew, whenever I needed a side dish for dinner. I've got two more 18' rows left in the ground ... the plants are dead but I'm told the taters are still growing in size even though their vines have died back.
I was ... negligent ... about weeding and the sod grew up around these potatoes so densely that you could barely see the vines. That probably cut my yield some. I didn't use any fertilizer except composted goat manure (and not a lot of it). We had plenty of rain this year and I put them in the ground in mid-May.
Not having grown red potatoes before, I had no idea what to expect, but this will probably be my benchmark for the future. Hope it helps you as well.