Use Compress HTML Liquid plugin.
The size difference for this blog having ls | wc -l
=> 188 entries many of which are not too large was ~2Mb of generated HTML. Although most older posts are doubled to support the older /YYYY/MM/TITLE slug format. So roughly 350 posts and pages.
The setup literally takes about 30 seconds…
_layouts
folder---
layout: compress
---
And you are done.
For more info visit the plugin’s documentation page