LL Cool J says Andy Ruiz broke the golden rule when it comes to STAYING successful -- he got comfortable.
If you know LL, you know he's a HUGE boxing fan (see: "Mama Said Knock You Out") ... and it's obvious he was disappointed to see Ruiz blow his run as heavyweight champ.
Ruiz told TMZ Sports he didn't take training camp seriously and "ate everything" -- which is why he showed up to the Anthony Joshua rematch 15 pounds heavier than their first fight.