Base (that's me, for infrequent readers) conducts an informal survey each season to find the player with the buiggest butt. Itt' really an homage to the late Kirby Puckett, who I belive is the gold standard. Any nominee will always be compared to Kirby. It was actually my wife that asked the question first. We were attending a Twins game (vs. Boston) in the mid 90's with a group of friends. A short serious discussion took place and we all decided that Tony Gwynn held that distinction at that time, although Rick Garces was also mentioned as a contender.
Over the past few seasons the distinction seems to fall amongst pitchers (Carlos Zambrano?). Perhaps it's just because we see them most of any position players on television, and in a wide variety of poses/movement.
Your vote, via comment, should not be confused with the Biggest Butthead in Baseball. That's a separate category where opinion contenders are Mark McGuire, Alex Rodriguez, and Ron Washington (actually Biggest Bonehead). Please join me in assembling these alternative statistics. I'll send the results to Bud Selig.