MP3 hasn't changed as a standard or probably as a codebase for decades. Your apps are changing every week, and apps tend to grow in size as more and more code & features are added to them. It's a natural consequence of most software projects.

The binary size of swift projects doesn't catch most people, only large projects with > 400k lines of code as they go over the cellular download limit. It's the new hotness, so many projects are adding swift even if the app is old.

Adding swift to a project adds ~20MB alone for the standard libraries that get included with each app, and each line of swift creates that much more binary bloat. The autogenerated interface between swift and objective C creates even more bloat.