Sir Mix-A-Lot -- Seahawks Win ... for Biggest Bootied Cheerleaders

The Seattle Seahawks have one HUGE advantage over the Broncos in the Super Bowl ... their cheerleaders' asses are wayyy bigger -- this according to bona fide booty-expert Sir Mix-A-Lot.

The "Baby Got Back" singer tells TMZ Sports ... there is no competition between the teams when it comes to butt-size, saying simply "Seattle cheerleaders got the best asses."

Mix -- who's from Seattle -- is a diehard Seahawks fan ... but his views on the cheerleaders aren't totally biased ... 'cause the rapper admits the "Denver cheerleaders have the best t**ties."