In open source, we need to manage our settings with our own module. Luckily, camptocamp/puppetserver provides exactly what we need! We can use this module to create a profile that applies to our Puppetservers:

class profile::puppetserver {

  class { 'puppetserver':
    config => {
      'java_args'     => {
        'xms'         => '4g',
        'xmx'         => '6g',
        'maxpermsize' => '512m',
      },
    }
  }

}