The screens alone are responsible for a lot of size increases (framebuffers in RAM, high res media) but also, unlike in Windows 3.11, modern Word allows you to

The screens alone are responsible for a lot of size increases (framebuffers in RAM, high res media) but also, unlike in Windows 3.11, modern Word allows you to mix English, Japanese and Arabic in a document, allows use of a screen reader, and has a thousand features that you personally don't need but everyone has some set of features that they use, and taking away any of them would offend someone.