I suspect a lot of this comes from OS X apps all shipping with most of the library dependencies, so there's no way of deduplicating libraries that are loaded in the memory. In Linux, shared libraries are used, at least with software installed by package management. OS X should benefit from a kernel feature something like Linux KSM (kernel samepage merging[1]), that scans for duplicate memory pages and merges them to shallow copy-on-write clones.

1: https://en.wikipedia.org/wiki/Kernel_same-page_merging