The width of a man under the arms is the same as at the hips.

A man's width across the hips is equal to the distance from the top
of the hip to the bottom of the buttock, when a man stands equally
balanced on both feet; and there is the same distance from the top
of the hip to the armpit. The waist, or narrower part above the hips
will be half way between the arm pits and the bottom of the buttock.

[Footnote: The lower sketch Pl. XVI, No. 2, is drawn by the side of
line 1.]

Vitruvius' scheme of proportions.