Typically a phrase used for typically novice bodybuilders or gym rats who only work upper-body giving them the appearance of a light bulb. They tend to over-compensate for aesthetics neglecting their legs sometimes entirely. This occurs until enough people have told them their legs look tiny, and they begin to do squats and other leg lifts.
"Have you seen how huge John has gotten?"
"Yeah but it's all upper body, he's suffering from human light bulb syndrome"