"I think I ate everything ... EVERYTHING ... that's why I gained so much weight."
That's Andy Ruiz keeping it all the way real about his loss to Anthony Joshua -- admitting to TMZ Sports he just let himself go leading up to the rematch.
"I was having too much fun, I was celebrating too much," the former heavyweight champ told us at LAX.
Ruiz says his partying was also out of control leading to the Dec. 7 fight in Saudi Arabia -- "Just being with my friends and just celebrating.