Config Backup: creates a zip file with your configuration settings Config + Graphs Backup: adds the data used to generate the statistical graphs (roughly +15MB)
In almost all cases one of the two options above is what you really want. The Full Backup creates significantly larger backup files (read - it takes much longer to create the backup and much longer to restore) and unless you know that you definitely do need that additional data, you should likely stick with one of the two options above.
Full Backup: adds on top of all that the replay / heatmap data
Depending on the amount of historic replay / heatmap data you have, a Full Backup can take a signficant amount of time and result in a very large archive to be downloaded. If you want to limit the data retained on disk, you can use MAX_GLOBE_HISTORY=365 in the environment variables on the expert page to only retain a year of data, this setting will not impact graph data.