(https://i.imgur.com/BP4VmQk.jpg)
The size of the Atari 5200 was always an issue and Atari figured they could do some cost cutting by making the console smaller. They had Mark Biassotti, the same person who designed the Atari 2600 Jr case, design the case for the new Atari 5100. The console was meant to be the same inside as the 2-port Atari 5200 and used the same TV switchbox as that model. When you look at the 5100 the one obvious difference is that it doesn't have a storage area for the controllers. I have to admit, this is a cool looking console. It's too bad Atari never released it.
(https://i.imgur.com/qMhsztQ.jpg)
Read more:
The Atari 5100 (Little PAM) (http://www.atarimuseum.com/videogames/consoles/5100/5100.html)
AGH MUSEUM:ATARI 5100 (http://www.atarihq.com/museum/5200/5100.html)