After riding Zwift all last winter and then riding outdoors in big group rides this summer, I would like to recommend that the drafting effect be increased in large Zwift pelotons.
In Zwift, I find that you have to produce around the same wattage in the draft as the rider on the front. If a rider on the front is doing 4 w/kg, then you have to do close to that to stay with the group (say 3.8 w/kg).
In real life, the lead/front rider can be doing 4 w/kg, but riders in the draft only need to be doing around 2.5 w/kg to 3 w/kg.
The result I find is that large groups in Zwift break apart too easily. This makes for constant regroups and unrealistic races. Just my two cents.