1
general::
Having broad hips or large buttocks. For example, I've grown too broad in the beam for these slacks. This expression originated in the 17th century and described the wideness of a ship. It began to be used for the human body only in the 1920s.
American Heritage Idioms
2
general::
Phrase(s): broad in the beam
1. Lit. [of a ship] wide at amidships. • This old tub is broad in the beam and sits like a ball in the water, but I love her. 2. Fig. Inf. with wide hips or large buttocks. • I am getting a little broad in the beam. It’s time to go on a diet. • John is just naturally broad in the beam.
McGrawhill's American Idioms And Phrasal Verbs