Why does Facebook have wide side borders on iPad mini?

Facebook does not have a native Mac app. It runs in a web browser. According to yourself, there is no issue on the Mac but there is on an iPad. Facebook runs on iPad/iPhone with an app written by Facebook. That tells me that the issue is with the Facebook app itself. Thus Jonathan Burger's suggestion to "ask Facebook" is a good one.

Ask Facebook.

Meta actually has a very poor track record for producing stable apps, especially when it comes to the iPad. That is why you see as much as 13 updates every month for Facebook as they attempt to resolve one bug and at the same time introduce another that needs resolved with a different update. It is clear with that update frequency that the users are actually the bug testers with very little in-house testing being done before an update is released.

Facebook has fired most of its experienced programmers (i.e, expensive programmers) so it can invest in AI development, and has replaced them with inexperienced programmers (i.e. inexpensive programmers). But even if they are good inexperienced programmers they need to learn and understand millions of lines of code developed over 15+ years, which is a huge challenge for any programmer (speaking as a retired programmer).

Ask facebook.

Apple does not make the application or provide the service.