Whoa, that shocked me as well:

WP 4.9.8:
zipped: 9.5 MB
unzipped: 30.8 MB

WP 5.0 Beta 1
zipped: 14.5 MB
unzipped: 49.4 MB

(I tested on macOS 10.14, and with a non-localized 4.9.8)

Welcome to JavaScript hell, I would say :-(

And for the plugin/theme compatibility:
Yes, WP 5.0 Beta 1 - with Astra theme for example has no in-post Meta Boxes anymore!!!

But: A WP 4.9.8 with Gutenberg 4.1 and Astra theme has the in-post Meta Boxes in the compat mode — like it was in the last 6 months or so.

This is another breaking change — I only hope that the Gutenberg, Core and Automattic teams reach out pro actively to Plugin and Theme devs/companies and inform about all those latest changes.

Otherwise we will have more than one big disaster in release week!

I for myself and all my clients won’t update existing installs to 5.0, the earliest will be a point release like 5.0.1 or so. And with „Disable Gutenberg“ plugin by Jeff Star or the alternative combo „Classic Editor“ / „Classic Editor Addon“ as the essentials.

The direction WordPress is heading for leaves me really really worrying.
Sad but true.