Executá-lo assim funciona. Acabei de adicionar --node-args="--max-old-space-size=6144"

MONGO_URL=mongodb://localhost:27017/btctestdb PORT=3000 ROOT_URL=http://app.backtocart.co METEOR_SETTINGS=$(cat /home/app/settings.json) pm2 start main.js --node-args="--max-old-space-size=6144" &