In a piece called "As Boys Get Fatter, Parents Worry One Body Part Is Too Small," Dr. Perri Klass assured parents not to freak out if the baby's member seems rather hidden.
"Questions about penis size have become more common over the past decade, as my colleagues and I have all seen more overweight children coming in for physical exams," she wrote.
While most of the penises that doctors see are of normal, proportional size, there is a phenomenon called the "hidden penis" that effects kids who are overweight:
The penis can be buried in the fat pad that sits in front of the pubic bone, and it can remain hidden as boys go through adolescence. What is called a “hidden penis” can be a combination of being prepubertal (so the penis has not begun to grow), being overweight (so the fat pad is significant), and in some cases an anatomical condition in which the soft tissue below the skin of the penis doesn’t adhere well to the Buck’s fascia, the thick covering that surrounds the penile nerves and arteries. This fixation problem can yield what Dr. [Aseem] Shukla described as a “slidey” penis, in which the actual shaft retreats and only the skin, or the foreskin, in an uncircumcised boy, is clearly apparent.