Shem

If you want that ornamental look to complement your house, then they're really cute bananas: absolutely nothing wrong with them.
I wouldn't've noticed their diminutive size had you not brought it to my attention. It's not as bad as it may seem. There's the optical illusion where the closer object appears to be bigger than the one behind. Next time take the photo with your hand behind the bunch and the bananas will look much bigger.
If you're concerned about edibility, then you could try a few things and still find out that's as good as it gets.
After all, Northern California isn't exactly optimum banana country.
If you're going for brag patch bunches you need to optimise the conditions so the plant can take up what nutrients you provide.
The growing media is within a confined space that is basically a planter box. It is possible that the media gets sour and needs liming more frequently; and/or needs replacing periodically, perhaps only one section each year ( just be careful of buried services).
The waxy cuticle on banana plants may prevent uptake of foliar feed. Some giant pumpkin growers intraveneously feed their plants. I don't know anything about this practice with bananas.
If that wall is facing southerly, then it is going to help keep the plants warm. It might also be drying the air. Some cultivars may tolerate lower humidity better than others.