Unfortunately, this seems to be something that the developers and product architects forgot to consider. Anyway, one way other than to editing the VM's configuration file manually is to use e.g. PowerCLI to do the job.
Connect-VIServer -Server <Your-ESXi-Host-IP-or-FQDN>
Get-VM "Your-vCenter-Server-VM-Name"| Set-VM -memoryMB 8192
André