Just because his 2001 debut, ''Room for Squares,'' sold nearly 3 million copies and snagged him a 2003 Grammy for Best Male Pop Vocal Performance doesn't mean Mayer, 25, is beyond humbling. In his upcoming video for ''Bigger Than My Body,'' the first single from ''Heavier Things'' (Sept. 9), he plays an ignored busker -- quite a stretch after a year of sold-out shows. ''I'm hooked up to these giant speakers around the corner, which everybody's running to hear, passing me,'' he says. ''It's a great way to suggest to people that maybe they don't know the whole story of what I can do.'' Like panhandle?