Constant 50mb+ updates to browsers is the new norm?

Can anyone enlighten me as to the cause of fairly constant browser updates (all of 'em, chrome, firefox...) that exceed 50mb per week?

Are there that many new features constantly? Are there that many bugfixes constantly? Are bugfixes related to recent features?

How are browser updates achieved? (binary blobs with known ABI's of some kind? Dynamic unshared libraries? Why doesn't my browser need to restart each time? etc.)

What are the driving factors of this "new norm"?