You: "Linux, on the other hand, must temporarily commit enough memory for the child process to be as big as its parent, only to then reduce to the size of the actual child. That temporary state would require lots of committed memory." –
man 2 fork: "Under Linux, fork() is implemented using copy-on-write pages, so the only penalty that it incurs is the time and memory required to duplicate the parent's page tables, and to create a unique task structure for the child."