Potato disapointment

I grew some International Kidney potatoes (Jerseys) for the first time this year, but have had disappointing results. They foliage grew extremely large (about 3 to 4 ft high/long) but only had about 2.5 lbs total of quite small potatoes from 12 plants  . On the plus side the do taste great 

This is in contrast to the Lady Christl that on average yielded around 2.5 lbs per plant. The Int Kidney were in a different bed than the LC and it was a newly dug bed that previously had some raspberry canes, but I fed it with Growmore and FBB before planting.

Does anyone have any thoughts why I got such different results from the two types?

The bed would, probably, have benefitted from some manure or compost; after the raspberries it would have been quite depleted.