"I think I ate everything ... EVERYTHING ... that's why I gained so much weight."
"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. Drinking a few more Coronas than I should have."
FYI, Ruiz weighed in at 284 lbs for the rematch -- 15 pounds heavier than his weight for the June 1 fight, when he shocked the world with a 7th round TKO over the British superstar.
Still, Andy tells us he was impressed with his performance despite being out of championship shape.
"Even out of shape, even the way that I was training back and forth (between Mexico and California) ... f**king, I did pretty good, dude! I did pretty good! But, I wish I had taken it more serious."
Ruiz lost via unanimous decision -- and even though he suffered a gnarly cut over his eye in the fight, he's in pretty good spirits. He also showed us the cut and it's already all stitched up.
Now, Ruiz says he plans to take some time off and hang with his family before trying to lock down a 3rd fight with Joshua ... a fight Andy says he'll PROPERLY train for.
"For the 3rd fight, I'm gonna give it all I got! I want those belts back!"