A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
The problem is Msft is clueless about design and never fixes their glaringly obvious mistakes. In this case, they are still using archaic tables to render page layouts which is something no one else has been doing for years. Even the humblest self respecting designer or developer would NEVER allow something like this out into production but Msft simply doesn't care.
I use bootstrap to better manage the responsiveness and look of blog pages, but to widen the post area you could probably get away with this styling:
table.ms-blog-MainArea, ul.ms-blog-postList {max-width:none !important;}
You may have to inspect the post area (using a browser inspector) to see if any other elements are constraining the width. Why Msft hasn't fixed this after so may years is beyond me.