@robert_knabe
As far as I know, no one is complaining about the small file size handling capability of pfSense.
The RRD files will still fit into the confiig file, and if logs are important (they are for me) an external syslogger shiould be used.
But, why not, if you want, you can modify the php.ini file, as that's the way to change memory available to PHP.
Two options :
Write a small shell script that modifies the memory_limit if it doesn't contain the right value for you. Have this script started with the startcmd pfSense package. Use commands like 'sed' to replace a text in the file. You probably have to restart the PHP-FPM to take the new setting in account.
Or, edit the php.ini file, and keep the old one.
Create a diff file from these two files.
Use the diff file and the pfSense patch package to patch the existing php.ini file.
You will still have to restart php-fpm to have the modification taken in account.
Both solutions will persist even after a pfSense upgrade.
You could add a feature request, just keep in mind that it would be granted only if a lot of people are asking for it.