If weeks actually passed (and you're not just being impatient), than they are probably nearing maturity, and I doubt they will get much bigger. Fertilizing in containers once the plants are grown is always a good idea. I add what would be considered obscene amounts of fertilizer in a normal ground garden to my containers to keep the plants happy. They could be poorly pollinated flowers, which give some blocky looking small fruits that have 1 or two seeds, it happens to me also at the beginning of the season if the plants are overly vegetative.

Stress. Either because of poor water availability, poor fertilization, or lack of sun. Not sure where you are, but extreme heat can lead to tomatoes that never get large.

Agree with all of the above and would add - too small a container as another major cause. Plus you don't give the name of the variety. Perhaps they are naturally small fruit. We need much more info please.

Dave

Thanks for your input! They could be heat stressed for sure!