1. Time
A host stops a script that works for too long, often after a few dozen seconds. A backup made in one go then stops halfway, and leaves an unusable file. A backup made in small steps, which picks up where it left off, doesn't hit this limit.
2. Space
To write a backup, you need space on the server, close to the size of the site. A backup that fills the disk can also get in the way of the site itself. Delete old backups you no longer need, or store them elsewhere.
3. The upload limit
To restore a backup from your computer, you have to send it to the server. Hosts limit the size of an uploaded file (PHP's upload_max_filesize and post_max_size settings), often to a few dozen or a few hundred megabytes. A larger backup is refused, sometimes after long minutes of uploading.
Three ways out: ask your host to raise this limit, send the file in pieces, or move the site from server to server, without going through your computer.
To free up space
- delete media that no page uses any more, and old page revisions;
- clear caches and temporary files, which have no place in a backup;
- don't keep old backups in the site's folder.