You might be thinking of the Sega Nomad, which was made explicitly to pay the full size cartridges. My dad had one for a while and it was the Game Gear turned to 11, flaws and all. Huge, hot, heavy, and devoured batteries. It was a cool concept on paper but nobody wanted to spend hundreds on it to play Sonic the Hedgehog on a tiny screen with a giant warm brick as a controller.