According the Unofficial 2014 NBA Census from BestTickets.com, the Memphis Grizzlies are the 7th shortest team in the NBA. They are also the 3rd heaviest in the league.
The difference between the Grizzlies' weight (227.14 lbs average) and that of the lightest team, the Sixers (216.13 lbs average) is only eleven pounds, but the extra girth definitely plays a role in the Grizzlies' ability to beat up and wear down opponents. Much of their size comes from Zach Randolph and Jarnell Stokes, both 6'9/260 lb players, but Koufos and Gasol each chip in another 265 lbs themselves. As they say when it comes to collisions -- mass wins.
Memphis is a shorter team simply because of their small set of wing players. They have the height where you truly need it with two 7-foot centers, but having 6'4 (Allen), 6'7 (Pondexter), and 6'6 (Carter) players as your most active small forwards bucks the NBA trend of having a freak athlete or two at the spot.