Limit throughput between Jenkins and Jenkins Agent
hmmm, tuff one.  dont think it's possible do do this directly
counter question, why do you wish to limit traffic ?  If you could do this, say through a network config of some kind, it owuld negatively impact build time.
From experience things that have an impact on traffic between agent and sever :

Artifacts: if you are storing artifacts within your builds they will be stored on the server. So at these points in your build process you will upload them from the agent to the server and depending on artifact size it will generate traffic.
Sync: Depending on what source control you use (if any) and how you use it it's possible that the sync to occur on the master and then transfered over to the agent.  You can probably change this so the sync only occur on the agent which will pretty much eliminate this part of the traffic between them.
Logs: all logs generated on the your jobs will be transfered back to the server.  Though to max out a connection here you'dhave to generate a whole lot of it !

It's posible some plugins can generate their own traffic.  Without knowing more on your tasks it's difficult to go further.
As to the actual question my first reflex here would be to look for ways to achieve this through a firewall or some other network system to throttle traffic.  Some will allow to target a specific application in addition to source/target IPs in their rules.