Re: Bananas grow smaller each year...
I have never grown them, and then stored them over the winter.
However, in my experience, if you want good output, you have to have good input. There is no substitute. That means sun, water, fertilizer (nutrition) and rich organic soil (lots of compost). Other issues are maximizing leaves (regardless of "looks") and thinning aggressive pupping varieties.
One or more of those things is lacking.