Certified GKH Gardening Expert
This can be attributed to a number of things, such as poor pollination, irregular watering, or the use of nitrogen feritilizer. A lack of pollinators can cause beans plants too produce less or develop poorly. Plants also need additional watering once they begin blooming. Since these are nitrogen-fixing plants, they do not need fertilizer, with exception to a side dressing of compost, if desired. An abundance of nitrogen will cause little to no flowering and poor pod development.
Also, could you be picking them too soon? If pods are harvested too early, they will not have finished their growth and the beans inside will be far too small.