Could be a few things:
Reduced pollen viability (less seed can mean smaller pod)
Excessive P can cause small pods and leaves
Extended cold nighttime temps (<55 F) can cause small pods
Deficiencies in N, P or Mg.
If the temps were very warm recently (during the pod's flower development) this could have caused reduced pollen viability. Another and less likely cause is insufficient pollination (not enough pollen collecting on pistil).
If you are sure weather was not the cause, you should test the soil pH (reduced uptake in nutrients) and to see if there is an excess of P or a deficiency in N, P or Mg.
Note: Even with feeding/watering regimens that are the same, due to the varying differences of each plant, the uptake of water and nutrients can be higher or lower. Also of note are differences in transpiration (water evaporation) from plant to plant which leads to a difference in the watering regularity which can change the pH of the soil (due to water mineral content and flushing of nutrients). All of this can over time, lead to varying levels of nutrients in the soil as well as the soil pH.
So even though you may be feeding them the same and watering them the same, over time the soil of each plant will start to differ in nutrients and pH which can cause all kinds of problems and confusion.
You can purchase online or at a garden center a soil test kit that prices around $14-$20 and has 4 different tests (pH, N, P and K) with 10 tests for each type. How it works is, you take a soil sample (I use a metal tube to take a "core sample") and you mix 5 equal parts distilled or bottled water to soil (5-1 ratio water-soil) and you mix this together really good and wait for the contents to settle. When you have a clear-ish liquid, you take the provided water suction tool to suck up water and squirt it into the proper test chamber up to the fill line. You then take a "pill" for the corresponding test (they are color coded) and empty the contents into the test chamber, reattach the lid and shake for a minute or so to mix the contents.
After about 10 minutes, the liquid will change a color which you can compare to the color chart on the test chamber, this is labeled like "Depleted, Insufficient, Sufficient, Excess" and you can get a pretty good idea in about 30 minutes as to the nutrient levels of your soil.
Here is a link to Amazon.com for "
Rapitest Soil Test Kit" for $13.55 plus s&h.
Hope that helps