I'm not sure why you're seeing this error, if it's due to the file being too large there are a couple of things that you can try:
1. Try appending the 'compact' flag to savefig:
This will prevent MATLAB from storing a compatibility layer which adds support for loading your figure in releases older than R2014b. The compatibility layer is quite large, so this makes a dramatic difference to file size, and also would work around any issues that are specific to this set of data.
savefig('mosaico.fig','compact')
2. Change your default mat file format, which can be done in the preferences window (under general). Setting this to version 7.3 or later will allow larger files.
However, the error reports a problem closing the file, which seems surprising for a large file. Perhaps something on the OS is using the file and MATLAB can't get write access? These sometimes show up for network drives, but it looks like you're targeting your desktop folder so I'd be surprised if there's an issue. But it still seems worth trying another a location?
