How this term came to life, is unknown. A commonly shared theory is that the Scandinavian "celeb" Frank R. Winsi all of a sudden started going to the gym, in order to get big. The only problem was, as previously mentioned, that F. Winsi was an ectomorph. Hence, he only gained mass to his arms. They got big, vast, enormous, huge, compared to his body size and height. He started on cheep russian steroids, and as a result, his body stopped producing testosteron on its own. In Scandinavia, its illegal with testosterone supplements, and therefore, F. Winsi became a girl. He lost all of his mass (it "transformed" into fat"). Frank then committed suicide.
With that being said, its important to remember that the story about F. Winsi is a myth. Nothing more, nothing less. But, that does not make the term "Winsy" less "real".
Words that basically mean the same: "Manlet" (short man with oversized muscles, like Mark Wahlberg in "Pain and Gain")