Notice that while the objects have a similar shape and size, the objects in the middle of the frame seem to be in a sandy knoll. This creates the perception that some are smaller than others, but those are at lower elevations, and are farther away from the camera.

This is the same image, it's been "inverted" what was dark now is light and visa versa. This helps to highlight the shadows, and identify what the shape of the objects might be. Interestingly enough, it also highlighted the shape of the "knoll."

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