You guys do know Kobe suffered a shoulder injury? (surgery done 10 months ago) and has NOT done any weight training/lifting since-- only conditioning.. Plus having an already fast metabolism.. After a month clearance of weight training this month->we will see drastic changes
Kobe is a skinny, lanky dude, always been that way.
This may actually be closer to the truth, we don't really have an idea when the photo was taken. I would also assume he's been cleared to do some sort of running/sprinting to keep his legs and conditioning.
All that said recent history shows when these guys retire they put on weight. NBA players are the most conditioned athletes among US professional sports.
All the conditioning keeps their metabolism up, if an individual already has a high metabolism in addition to the "cardio" they're doing putting on and maintaining muscle would seem almost impossible. (This is why I called BS on LeBron putting on size during the NBA season in Miami, when he would routinely ride his bike to and from the arena/ although that may have been pure hyperbole).
Being in the NBA I'm sure they have access to means that a regular person would never get, like the best training and recovery methods and nutrition.
Kobe simply may have lost weight from inactivity from the torn rotator cuff, but that alone isn't looking at the whole picture.