I don't know if Cisco has published any document that tells you exactly what goes on between these two systems, or how much data is exchanged. My advice would be to run a tcpdump on both ISE nodes that have pxGrid enabled - and set a filter to only capture the traffic to your DNAC (probably the Enterprise VIP, or whatever your setup looks like). Run that for 60 minutes and then run it through Wireshark to see IO stats and other cool charts. It will tell you everything you need to know. The ISE tcpdump filter to capture only the traffic to and from 10.1.2.3 looks like this:
ip host 10.1.2.3
.