Some types of tomatoes are small naturally. Perhaps your plants are not the varieties they were tagged to be. Stress such as drought can make a variety produce smaller than normal fruit but your description of care seems to eliminate this possibility. I think the plants have lost foliage due to fungal leaf diseases or spider mites over the course of the season. Both can be sprayed for but at this late date such spraying is not needed. The plants don't have much season left and there won't be a benefit from spraying them now.
Q&A Library Searching Tips