Andy Ruiz might want to think twice about ditching his fat ... cause "The Biggest Loser" trainer Dolvett Quince tells TMZ Sports he might advise the stud boxer against it!!
"I think the weight that he had on was to his advantage," Quince says ... "Maybe the blows didn't hit as hard."
Of course, Ruiz -- who's got a huge rematch coming up against Anthony Joshua -- doesn't seem to be listening ... he appears to be dropping lbs by the day -- check out this vid he posted earlier this week!