vCloud Director 8.20 allows deployment of Org VDC Edge Gateways in 4 different form factors from Compact to X-Large where each provides different level of performance and consumes different amount of resources.
As these Edge Gateways are deployed by NSX Manager which allows setting custom reservations for CPU and RAM via an API call PUT https://<NSXManager>/api/4.0/edgePublish/tuningConfiguration, it is also possible in vCloud Director to set custom reservations.
Why would you change the default reservations? Reservations at VM (Edge) level reserve the resources for itself which means no other VM can utilize them in case they are unused. They basically guarantee certain level of service that the VM (Edge) from performance perspective will always deliver. In service provider environments oversubscription provides ROI benefits and if the service provider can guarantee enough resources at cluster scale, than the VM level reservations can be set lower if at all.
This can be accomplished by tuning the networking.gatewayMemoryReservationMultiplier and networking.gatewayCpuReservationMultiplier settings via cell-management-tool from vCloud Director cell. By default the CPU multiplier is set to 64 MHz per vCPU and the Memory multiplier to 0.5.
By default Edge Gateways will be deployed with the following reservation settings:
The following command will change memory multiplier to 10%:
/opt/vmware/vcloud-director/bin/cell-management-tool manage-config -n networking.gatewayMemoryReservationMultiplier -v 0.1
Note: The new reservation settings are applicable only for newly deployed Org VDC Edge Gateways. Redeploying existing edges will not change their reservation settings. You must either use NSX API to do so, or modify Org VDC Edge Gateway form factor (e.g. change Large to Compact and then back to Large) which is not so elegant as it will basically redeploy the Edge twice.
Also note that NSX 6.2 and NSX 6.3 have different sizing of Quad Large Edge. vCloud Director 8.20 is by default set for the NSX 6.3 size which is 2 GB RAM (as opposed to NSX 6.2 value of 1 GB RAM). It is possible to change the default for the reservation calculation by editing networking.full4GatewayMemoryMb setting to value ‘1024’
Tom Fojta's Blog 
Hi Tom,
It looks like the default reservations could be a bit higher in some versions of NSX.
For example NSX 6.2.5 and newer release notes, list that a compact edge gateway would receive a 1000Mhz CPU reservation instead of 64Mhz.
http://pubs.vmware.com/Release_Notes/en/nsx/6.2.5/releasenotes_nsx_vsphere_625.html#upgradenotes
Can you clarify if these higher reservations also apply to new edge gateways deployed by vCD – if you are running NSX 6.2.5 or newer?
Note that changing the reservation of an existing NSX Edge is done using the NSX API, but can you tell which API and how to use it?
https:///api/4.0/edgePublish/tuningConfiguration
It is now also available in the NSX UI.