Not enough free space?

This looks like an Apple bug. MacOS/iOS have never been great at determining how much space is available on an iOS device, but recent versions of the OS seem to have lost the plot completely.

I'm trying to sync music files from Catalina to an iPhone 7 running iOS 14.4.1. According to the phone, there is 92.27GB free on the phone. The Mac estimates the free space as 92.47GB, which is close enough.

I have a small number of playlists that I have already sync'd to the phone. If I choose one additional playlist, which contains 28 songs (MP3 format, so perhaps 250MB in total) and try to sync, the Mac tells me that there is insufficient space and that an additional 50GB will be needed. In short, these 28 additional songs apparently require a total of more than 140GB of space on my phone. As you might imagine, I find this difficult to believe.

One possible clue is given by the fact that when I try to sync this 28 song playlist, I also get a warning about a number of songs that were not copied because they are not authorized for my Mac (these are iTunes purchases that were bought on the same machine before I upgraded the OS). None of these songs that I am being warned about feature in any of the playlists I am trying to sync, either the ones already sync'd or the one additional playlist. This suggests that the problem might be that whatever algorithm Apple is using to determine which songs need to be sync'd is building a completely incorrect list, leading to miscalculation of the space needed.

Authorizing the computer to play the purchased songs causes the "not authorized" warning to go away but doesn't resolve the space issue.

It looks to me as if you (and I) have encountered a genuine bug in the way that the OS calculates which files need to be sync'd and how much space they require. I haven't yet found a workaround, which means that we may need to wait until Apple acknowledges the problem and decides to issue a fix.

UPDATE: Just for amusement, I created a new playlist and added 1 song to it, then tried to sync. Once again, the OS tells me that I need an additional 50.85GB to sync successfully. Actual size of the file in question? 10MB. Space required on the device? 140GB. I'm starting to think that math isn't really Apple's strong point.

I've done some more digging into this, and I think the problems I've been having may be caused by using nested Smart Playlists, i.e. a Smart Playlist that selects files from another Smart Playlist (or possibly a regular playlist). Sync seems to have trouble coping with nesting generally.

Sync does seem to be horrifically broken in Catalina. Very little of it seems to work the way it should. It's as if someone at Apple said "Here's this hugely important functionality that we're moving into the Finder for the first time, why don't we find the newest summer intern and have them implement the whole thing?"

OK, maybe I exaggerate. But it's ... not good. Playlists that are checked may or may not sync, syncing folders can result in the creation on the device of a folder and a smart playlist that has the same name but potentially random contents, and so on. It's just chaos.

If anyone else is struggling with this, some things I've learned include:

a. Don't try to sync Smart Playlists that reference Smart Playlists.

b. If a playlist doesn't sync, uncheck and then recheck the checkbox.

You can't store more data than you have space.