Small underdeveloped fingers can be caused by environment, less than favorable growing conditions will cause stress and reduce yield for example cold temperatures, low lighting, too much or too little water are just some of the factors that can affect bunch weight. There is a direct link between K (Potassium) and the amount of hands, size of fingers and the time it takes for a bunch to mature. That being said adding more K does not always equal larger bunches because there are other factors that can reduce K uptake and affect bunch weight.

I am not a cold weather basement growing banana expert, to me they look fine and I would follow Migael's advice leave it alone unless you start to see signs of mold.