I have just reached 2000 reputation, and since I have used this website for a while, I noticed that my homepage had changed format. I don't know how this happened. Is this a specific system feature or something else?
It seems you've accidentally tripped the mobile version of the site. That setting is stored per-site in a cookie, so that's why you still see the full version of the Meta site.
To go back to the full version, scroll down to the bottom and click the 'Full site' link: