The order of sizing assumes that you're using all of the features of the Edge (LB, VPN, FW, Routing), and scales pretty linearly:
Compact = 1 vCPU / 512 MB
Large = 2 vCPU / 1024 MB
X-Large = 6 vCPU / 8192 MB
The sizing of the X-Large assumes that you're doing a lot of L7 load balancing. Quad Large is its own special thing that falls outside the normal order. The idea is that it is the same as a Large in most respects, but augmented with 2 additional vCPUs for additional throughput for straight forwarding and firewalling without a lot of L7 load balancing going on. It was a common enough use case that they added it in.