It is updated only if new textures are added to the game. Not on every new BAB version.
Why is the download that big? Does the addon need that much memory?
No, it does not. BAB Bars depends on custom versions of all icon textures to work. All borders of all textures must be translucent. This is not the case with the default wow icon textures. This download contains modified versions of all standard icon textures. A big download does not mean that the addon needs that much memory ingame. The additional textures a loaded on demand only. The will not significant blow up the memory usage by default.