Since that is the case, it is plausible that the numbers themselves aren't the culprit, but the base capturing that goes on with that amount of people on.
Wouldn't even necesarily have to be a 'mass' goon drop- figure with 500 people on there are usually 5-6 big attacks going on, plus 30-40 milkruns on one of the big maps. Probably have 4-5 sets of troops 'in the air' at any given moment- with 500 people on thats 25,000 messages the server is trying to send out all the time.