I'm getting sweet 100s the size of my pinkey mail, when they often get the size of my first thumb joint. Could square foot gardening, and being shaded too much by the cultivars Big Zac/Mushroom Basket be causing this?
My answer is based purely on my personal experience, in a garden where the beds march toward more and more afternoon shade, so I can clearly see the effect of shade when I plant a few beds in the same thing. Tomatoes in the beds with more afternoon shade have both smaller plants and smaller fruit--shade has always caused both problems for me.
So if both plants and fruit are small, yes, shade seems like a reasonably plausible cause.
If your plants are big and lush but the fruits are small, I would put higher odds on the cause being too little water or pollination problems.
Officially, too much nitrogen fertilizer should also cause big plants and small fruit. I've just never seen that happen.
I've grown a lot of tomatoes in close proximity. Those that are shaded by other tomatoes don't tend to have particularly smaller fruit than the non-shaded portions in my garden. However, being smothered by another plant can seriously impact production, and shade can impact the taste and production.
I'm not saying the fruit size is never affected (it probably is, at least sometimes), but it doesn't seem to be the rule in my experience.
I would personally suspect the soil and other growing conditions to be the culprits. If your soil is high in nitrogen or low in potassium or magnesium that might explain it.
If there isn't enough available soil, that could definitely cause it. You say you're doing square foot gardening. If the soil only goes so low, instead of allowing the roots to grow as deep as they want, that might be the problem.
As others have said, excess nitrogen can cause a lot of leaf growth and less fruit. However, excess nitrogen is also known to cause smaller and more fruits (but not more fruit by weight). I'm not sure what conditions lend to each, though.
Potassium influences fruit size; potassium is said to be less available when it's hot. Some people think phosphorus influences fruit size, too (but it's more available when it's hot, I think I've read).
Magnesium can affect heat-tolerance (and plants are sometimes known to have smaller fruits in the heat). Magnesium is also less available when it's hot. Some people think magnesium influences fruit size. I wouldn't be surprised if your soil is quite hot (temperature-wise).