Do the small ones all coincide with the watering heads?

I actually haven't been using the micro sprinklers. Watering by hand gives me something to do during Corona time. Everything gets even watering.

Maybe the smaller ones were planted a bit deeper, sprouted a bit later and are now playing catch-up. Or maybe it's like kids....one's tall, the next one isn't. That's sure going to be pretty when they all bloom. I planted two in pots, and something ate them to the ground. I'll have to start over. You inspire me.

Be aware that sunflowers are allelopathic. Every part of them. They inhibit the growth of many kinds of nearby plants. Interestingly, they inhibit the growth of sunflowers as well. That's called self-inhibiting or autotoxic. So if you plant sunflowers next to each other they'll try to suppress each other.

They all sprouted at the same time, the small ones haven’t changed in weeks, they just sit there. Daninthedirt, I’ve never heard that before. How to explain the fields of sunflowers I’ve seen in Chile and France? A quick google search shows some research suggesting sunflowers can inhibit other plants in a subsequent growing year, but I found nothing indicating that sunflowers inhibit other sunflowers when growing side by side. I’ve grown lots of sunflowers in my gardens over the years but I’ve never seen any allelopathic effects.

Just Google "allelopathic sunflowers". You'll find lots of info. It's well understood. Sunflower extract has been investigated as an organic herbicide. Some plants are affected more than others. Potatoes and beans, very much so.

Autotoxic? For example

https://courses.missouristate.edu/pbtrewatha/annual_sunflower.htm

Many references. As to fields of sunflowers, each one would probably do better if the others weren't there.

I've also seen fields of sunflowers in Lompoc where they grow flowers for seeds. The article you referenced is for a common sunflower. Perhaps it is not as true of hybrids.

Could it be the direction of the sun? do the tall ones shade the others?

As I said, with autotoxicity, plants would simply do better if there weren't a lot of the same plant nearby. That doesn't mean they can't grow. BTW, strawberries, cucumber, and even alfalfa also show autotoxic behavior. Their root exudates are not good for the same kinds of plants. Even so, people commonly grow them with plants bunched up in beds. It's well understood that taro should not be grown in consecutive seasons in the same bed for exactly that reason.

Hydroponic gardening is advantageous for at least strawberries, because with the right methods of recycling the growth water (as in, filtering with activated charcoal), those allelopathic chemicals can be removed.

Loads of research papers on all this.

Now, clearly, allelopathy can be very helpful to plants. Not clear how autotoxicity can, however. I need to ask a botanist about that. May be that it's a form of auto-thinning, where growth of young plants is inhibited to increase the available nutrients to more established plants.