I started my squash in pots this year to get a head start. I put them in the garden within a week of germinating when the first true leaves were just starting to open. All are doing fine. My zukes and hubbard look about the same size as your zukes. I can see little 2" zukes forming. And my patty pan squash and vegetable spaghetti are smaller - about the same as your yellow. Butternut, acorn, kabocha and pumpkin are all about half way between the 2 sizes.
I can't figure out why. They all have the same garden soil, same compost. Maybe its a location thing.

'.....a small amount of powdery mildew...' Can you post a closer picture? From what I can make out it could well be just the natural silvery patterning some zucchini get.

I was going to say the same thing as flora just mentioned. Some squash leave have silvery shade of spots.

But your YelloW Squash !!! I see two automatic sprinkler just next to it !!! I suspect too much water leaching ot the nutrients beyond the reach of the roots. Try to cut down on watering and feed it. If those wood chips consist of a thick layer and mixed with soil and you are fertilizing from surface, Then the wood fibers are absorbing the nitrogen.
The other reason could be partially rotted root system.
IMO

Your yellow squash plant looks like my squashes and cukes when they were first transplanted. They had been in cups on my deck which were watered almost every day without being fertilized, except once in the beginning, so the nitrogen was pretty effectively washed out of the soil and they were yellow. I gave them a foliar feed and fertilized with a granular organic fertilizer at planting (as well as adding plenty of compost) and once their roots got into the garden soil they took off and greened up nicely. If your yellow squash was root bound that may have made it harder to spread its roots into the soil. In any case, it looks like it needs nitrogen.

Seysonn, We had a very wet June in NJ this year and I don't recall watering my garden at all, so nothing I could do about that.

Would feeding it Tomato Tone be a bad idea to give it a Nitrogen boost? How much would be sufficient? Normally I use 3 tablespoons around my tomato plants (and half that amount around my pepper plants) twice a month.

For a quick boost, you might consider a liquid fertilizer. Tomato Tone and similar products release nutrients more slowly.

I actually just bought a liquid fish fertilizer for my annuals the other day and boy did they like it. Never considered using it for veggies. Should I? Nasty stuff.

I use it on my veggies, the smell goes away soon. But the plants love it.

The floods washed the nutrients from my broccoli bed and left it with purple, dying leaves. A couple doses of liquid fert brought it right around.

Whether the heads will be worthwhile or stunted is another question, but no more purple leaves.

Your squash needs feeding. Mine looked that way, I sprinkled a small handful of 13-13-13 on top of soil and watered it into the ground. You'll see it turn bright green in about a week.