A picture would help...have you ruled out herbicide drift or contaminated compost?

I have read about herbicide drift...and I suppose that is possible too. Basically, I just have all very small leaves, one-third normal sized (not twisted or really distorted)...on the top half of my plants.

Pictures would help. But another possibility in addition to a virus or herbicide is little leaf disorder.

Unfortunately, I am old and a bit technologically challenged on attaching pictures. But, as I look at online pics of "little leaf"....it sure looks similar to what I have. It sounds like not much is known about that disease. hmmm

If you can take a digital picture and trnasfer to your computer, no need for technical stuff to upload the image -- Just click the mini camera underneath/below the text box.

Or ask someone to help you, be that a friend, neighbor, relative, or 16 year-old neighborhood kid/grandkid.

Here are pics (I think)

First picture looks rather good. Maybe in second picture the plant closer to the camera seems to have a problem.

Kinda far to say for sure, a virus would be also somewhat stiff upper growth. There are also micronutrient deficiencies which can cause small upper leaves. That plant also seems to have an unusual chlorosis going on. If it's the only one with those symptoms, better to just remove it.

all those signs do point to TMV, but it's still hard to tell by distance in the photos.
are any of the fruit developing any spotting?

https://www.extension.umn.edu/garden/fruit-vegetable/plant-diseases/tomato-mosaic-virus-tobacco-mosaic-virus/index.html

It certainly appears that several viruses have similar effects on top growth of plants. CMV, TMV, and TOMV all appear to have a stunted/small leaf production. "Little Leaf" disease seems similar...but I am not sure it is in So Calif. hmmm

Interestingly, my pumpkins in an adjacent bed has now become diseased with what looks like...CMV! The pumpkin leaves have now started to become blotchy shades of yellow and green. The more advanced diseased leaves have died and turned brown.

CMV usually causes shoestring leaves in tomatoes, not just small leaves. Of course, diseases don't always present typically.

Something to keep in mind: -pyralid herbicides cause blotchiness in squash foliage, and the leaves later die.

My pumpkin leaf next to my tomato bed. Eventually, this leaf will completetly turn yellow/brown and die.