According to the photographer, Lewis Hine: "Group of boys working in Great Falls Mfg. Co., Somersworth, N.H. 6 p.m. May 18, 1909. Smallest boy is Alfred Ouellet, 212 Main Street. Fat boy is Willie Laudry, 35 South Street. Boy on left hand end is Napoleon St. Lawrence, 23 Union Street."
I think by "fat boy" Hine means the kid on the far left, who would hardly be fat by today's standards. It is a reminder of just how skinny most of these street kids and child laborers were.