I just tried to sync 1.7 GB of files onto my girlfriend's Z710. It has a 2.0 GB memory card with 1.8 GB free, so this should not be a problem -- especially since I've configured an external encoder (Nero Digital) to convert the music to a considerably lower bit rate, so the final size should be around 1.0 GB, leaving a lot of space free.
However, MC told me that I'm 9.9 GB over the limit (I think, possibly it was 9.7 GB or something). In other words, my 1.7 GB, which will shrink to around 1.0 GB, requires all of the 1.8 GB plus another 9.9 GB...
Needless to say, something is wrong here. I accepted the suggestion to sync what would fit and remove the rest, which resulted in an unknown amount of music being converted to 91 MB and copied to the phone. Now that I'm about to sync again, MC tells me I'm only 8.3 GB over capacity. Most likely I'll be able to transfer everything, but it will take a number of rounds and I'm not too keen on going back to the computer every 15 minutes (which it takes to convert one batch) just to click "sync" again.
In any case, the reason for this may be that since I'm converting the songs and I'm using an external encoder, MC does not know the final bit rate (it obviously can't parse the command line to the converter I use) and consequently it assumes CD size at 176400 bytes per second. This at least seems to fit the numbers very well.
I'm not sure what the correct solution would be since you obviously can't know what size songs will be after conversion, especially if some form of quality-based setting is used rather than a bitrate setting. Perhaps just keep transferring songs until the memory card is full and *then* start complaining to the user?