I don't know if this is relevant or not, but Safari uses OSX's ported libraries as DLLs. If the memory size of those DLLs is being used, it would show up as les

I don't know if this is relevant or not, but Safari uses OSX's ported libraries as DLLs. If the memory size of those DLLs is being used, it would show up as less memory usage on an OSX machine due to those libraries being a shared part of the main OS.