This message means that the zip engine creating your backup had to stop, because the system it is running on is configured with a maximum file size limit.
i.e. The zip file created by that zip engine isn’t allowed to be larger than the limit.
This is very rare – we have only come across one hosting company that this occurs with (Strato, in Germany). To avoid it, tell UpdraftPlus to use a different zip engine. (The limit appears to only apply to one of the three possible zip engines).
To do that, edit your wp-config.php file for your WordPress site in a text editor, and add this line somewhere near the top (after the opening <?php) :
Then, start a fresh backup. You should not see the message again.