I recorded a minute's worth of heavy traffic with tcpdump.

 tcpdump -B $((100*1024)) -i eth0 -n "port 27017" -w /replay/test2both.pcap

The resulting pcap file was ~8GB. I then processed the file using:

mongoreplay record -f test2both.pcap -p test2both.bson

The machine, a c4.8xlarge, had 64GB of memory, about 55GB of the which showed free using top when the mongoreplay record command was run. Little by little, free memory declined until the process crashed with:

fatal error: runtime: out of memory

Link to the pcap file and full mongoreplay output from crash in first comment.