It's sort of a controlled experiment, with the idea that you don't seem to be doing anything to either plot. But the hugel plot has the benefit of a sort of implied weeding (through mulching), where the other plot isn't being weeded at all. I wonder if a better experiment wouldn't have been to mulch the standard plot, so that what you're comparing is two mulched squash seedings, one on hugel and one on tilled ground?
At any rate, it shows the benefit of hugelkultur from a hands-off (after planting, at least) method.
Also, don't discount the possibility of considerable soil variation within even a small space. I have squash plants, interplanted with field corn, that are perhaps 10' apart but that show similar variation as in your photos (the corn, too). There's just that much difference in soil fertility.
There is definite signs of nutrient deficiency in the "standard plot" along with more weed competition. My guess is nitrogen. My guess is that your hugel bed is already being broken down or included some nitrogen-rich stuff to off-set the ton of carbon hugels usually have. I should also mention that in my experience, however, squash LOVES compost and usually doesn't shy away from hanging out in something half decomposed where as some other crops might suffer a little.
That all said, I think this little anecdotal experiment shows that for you, this year, your squash is happier on hugel. Good job and thank you for sharing. If more people do this and share here (both the good and the bad), we could move from anecdotal to clinical trial.
I have some squash I grew on leaf compost covered w/garden soil and straw bales covered with garden soil. The leaf ones looked very pathetic and died. The strawbales looked a little pathetic, were fed calcium and took off. Now they have conquered my fencetomatoes creating one of the many areas of chaos that I forage for food in. On the other hand, I planted some gourds at the other end of the yard with some compost or "garden soil" next to it, the seed going into normal soil. They thought about what they were going to do for a while, then took off and are creating chaos where they roam. On the side yard I have some other unknown squash that received some garden soil and a similar (or so I thought) treatment but are more or less pathetic and going no where. Same in the front. This could be related to the types of squash- the gourd, zucchini, and butternut might just be overall better adapted than the pumpkin and watermelon. However, two years ago I grew one squash almost in a compost pile and another away from it. Compost baby went butter-nutts and the other was butter-pathetic.
Oh, I should note I live in a generally wet environment, rain usually 1/week and I do water if I see something dry.
May I take this post a little further and ask for opinion on placing in a 60cm raised bed straw bales still tightly bound bit of old wood and leaves and manure and then topping with a 20 cm layer of good soil. My belief is it would be very cost effective to do this and it would also hold water ++++ which would negate the need for everyone to turn to wicking beds. Do not get me wrong I like the wicking bed idea but I think it is an expensive exercise which has alternatives. Thanking you in advance