Why the BMI is related to the size of the head, ? Which affects uses of helmet?

Why the BMI is related to the size of the head, ? Which affects uses of helmet?

Based on the following reference:
Romanow, N., Hagel, B., Williamson, J., & Rowe, B. (2014). Cyclist head and facial injury risk in relation to helmet fit: A case-control study. Chronic Diseases and Injuries in Canada,
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1.- Why the BMI is related to the size of the head, which affects uses of helmet.?