Mario Kart XXL was a Game Boy Advance game being developed by Denaris Entertainment Software. The demo was intended to show how the Game Boy Advance was capable of using multiple layers of Mode 7, the technology used for rotating and scaling rendered backgrounds in various Super Nintendo and Game Boy Advance games, including Super Mario Kart, F-Zero, and many others.
There is no copyright info anywhere in the game except for a screen with the Denaris Entertainment Software logo when loading the game, meaning that no one truly knows when this was even made. It's been speculated that this demo has existed since as early as 2004, seeing how the title screen uses artwork taken directly from Mario Kart: Double Dash!!, and use of a GameCube with a Game Boy Player is required to return to the main menu by pressing "X" (none of the GBA models had an X button). Both Mario Kart: Double Dash!! and the Game Boy Player were released a year prior in 2003.
It's worth noting that this unreleased tech demo is one of the only GBA games to ever make use of multiple Mode 7 layers.
Word got out that this demo existed in August of 2015 when a YouTube channel dedicated to rare video game prototypes by the name of "Steven Seventyeight" found the cartridge and recorded footage of it being played on a GameCube via the Game Boy Player accessory.
The sound quality is rather low, due to the amount of rattling coming from the camera used to record this footage. Unlike dbxbeat's low-quality Socks the Cat video, this is unintentional as further inspection shows that the rattling is in all of Steven's videos, due to the camera he uses to record videos.
Note: Steven states in his YouTube channel description that he doesn't release ROMs of any of the games he finds, so don't try to ask him for it. It might be possible for him to sell the physical cart on an online auctioning site, but that's also very unlikely seeing how someone could just dump the ROM and then release it online, something that Steven clearly doesn't want.
Mario Kart XXL has a "Demo" and "Play" mode. Demo mode plays an automatic playthrough of the tech demo that repeats, similar in vein to the Super Guides from the New Super Mario Bros. series. Play mode allows you to play through the tech demo. There is no character select; the only character available is Mario, who is pre-rendered with a custom 3D model. You can return to the main menu at any given time by pressing the X button on a GameCube controller during gameplay.
Mario Kart XXL consists of a time trial with 3 laps. The game starts you out on an unnamed track reminiscent of Donut Plains from Super Mario Kart, with a second layer underneath the track depicting a beach-like area similar to Koopa Beach, also from Super Mario Kart. Coins appear on the track, but can't be collected.
Mario doesn't slow down when exiting the track unlike other Mario Kart games. It's possible to fall out of bounds, respawning you on the track after hitting the bottom layer with a splashing effect, albeit without the help of a Lakitu. Transparent windows on the track that show the bottom layer are present towards the beginning and end of the track. The camera zooms in and out at times, but you can control the camera manually by using the L button to zoom out and the R button to zoom in.