Cherry Sized Heirlooms!

First year grower here! My heirloom slicer tomatoes are ripening at about cherry tomato size – can the r/tomatoes community help me identify why? Here are a few specifics about the plant:

• Burpee Heirloom Slicer purchased from home depot as a seedling and transplanted in late May, after Memorial Day. Would have liked to start earlier but this is when we moved into our house.
• In probably too small of a pot / too little soil. Less than 1 cubic ft. See last pic for pot.
• Zone 7a with hot dry Summer with highs consistently in the upper 90s. I did not have a shade cloth.
• I did not fertilize when I transplanted, or until after fruit was already showing (July). The plant was transplanted into Miracle Gro fruit & vegetable soil and the fertilizer I eventually used were Jobes organic tomato spikes.

The upside is that the taste is still good, but not exactly the yield I was hoping for. Would love to hear everyone’s thoughts on how to improve next season!