So you trimmed the plant back far enough that it still isn't bushier this year than last? I'm not one who likes to trim back overwintered plants any more than necessary for storage purposes plus getting rid of any dead limb areas in the spring.
Are the temperatures higher this year? I'm wondering if they're stunted by that and not enough room for new root growth as a 30cm diameter and 20cm high pot seems small for an overwintered plant, I calculate that if completely full of soil and same diameter at the bottom (not smaller as most pots are) that's only 3.7 gallons.
Being root bound can have two affects. One is a smaller overall plant size and this reduces # of pods produced. The other is lower nutrient delivery which through fertilizers you can improve but if it is not improved (enough) then you may also have smaller pods. Is the plant in the same place so it receives the same amount of sun at the same time of day?
I do sometimes notice that the further away from the main stem a pod grows, the smaller the size it ends up. I also notice smaller pods if a plant is starving for nitrogen but then the leaf growth is a bit stunted too.
Measure the temperature of the pot in direct sun. If it's not getting over ~ 95F then I doubt the pot material is a factor, though a darker color could be getting warmer too.
Back to the topic of trimming them back, if that is what happened I'd overwinter and trim back much less. Even if the pods stay smaller you should end up with more volume in total given a big enough pot.