When restoring from USB during install, if the config file contains RRD data, the final config.xml on the system will also contain all the RRD infomation
When restoring from USB during install (according to https://doc.pfsense.org/index.php/Automatically_Restore_During_Install), if the config file contains RRD data, the final config.xml on the system will also contain all the RRD infomation.
It's going to store all the RRD information which was present at the backup time, between the <rrddata></rrddata> tags at the end of the file, and this file will be the main config file of the system as it is.
RRD information is not restored though.
Configuration saves take longer because of this.
Backing up the configuration from the web interface using the default "Do not backup RRD data (NOTE: RRD Data can consume 4+ megabytes of config.xml space!)" ticked, will still result in an XML file containing the RRD information.
Purging manually all the data between <rrddata></rrddata> and the tags themselves, then restoring the config from the web interface will reboot the firewall, RRD data will still be lost, but at least the config file size will be back to normal.