If you search for and download a "100MB Office 2013," you are almost certainly getting one of three things:

For context, early versions of Microsoft Office were tiny by today’s standards:

The explosion in size comes from high-resolution assets, complex XML parsing engines, cloud sync features, and security libraries. You cannot put Office 2013 back into an Office 97-sized box.

Sometimes, these files are "hacked" portable versions. Modders strip out help files, spell checkers, themes, and sometimes entire features to make the program tiny. While this might sound like a good trade-off, these versions are notoriously unstable. They crash without warning, they don’t save your settings, and because they are hacked, they often lack critical security updates. Using one for important work is a recipe for disaster.