The INSTALL file from emacs-26-rc1, as well as from master, talka about disk space requirements for emacs 25.
Should that be updated?
1. Unpacking the Emacs 25 release requires about 200 MB of free
disk space. Building Emacs uses about another 200 MB of space.
The final installed Emacs uses about 150 MB of disk space.
This includes the space-saving that comes from automatically
compressing the Lisp source files on installation.