Not sure what you're looking at, but Bad Rosemary's head was rather small compared to her neck and body, as was Shady Lady, who looked almost pin-headed to me.
The Queen had an average head IMO, but certainly wasn't bucket-headed or exaggerated in her head size. Zebo was also about average for a male.
Banjo had a pretty good-sized nugget on him, I agree, but Boomerang had an average (to slightly below-average) sized head on him, for a male, IMO. I do agree that Chinaman's head was smaller than average.
Therefore, of the 7 hard-mouthed dogs you yourself just mentioned, I would consider 1 (Banjo) to have an above-average sized head, 2 (Zebo/Queen) to have average-sized heads, and 4 (Boomer, Bad Rosemary, Shady Lady, and Chinaman) to have below-average sized heads ... which falls exactly in line with what I just finished saying above: " ..of the hardest biters I've had, they were a little on the average/thin-headed side. A little."
Jack