Apple has already announced the successor to Leopard, called Snow Leopard, during the WWDC not too long ago.
They explained that Snow Leopard would not focus on user-visible features, but instead would deliver performance improvements and resource footprint reductions.
One of the measures Apple has taken is the size reduction of application binaries, which has resulted in dramatic weight loss for a lot of applications.
AppleInsider has found out what exactly Apple has been doing to lose that much weight.