For most applications the machine code is not the bulk of the size, that’s assets like images. These aren’t duplicated in a ‘fat’ binary so the size increase is

For most applications the machine code is not the bulk of the size, that’s assets like images. These aren’t duplicated in a ‘fat’ binary so the size increase is rather modest. The advantage is convenience for the user (and developers), one download that works transparently for all computers.