huge difference in RX bytes vs TX bytes (ifconfig) on Syslog-NG server

To be clear, RX and TX are Receive and Transmit (not transfer).
Ie, traffic sent to this host (RX on its network interface) and sent from this host (TX on its network interface), respectively.

      RX packets:60451021996 errors:0 dropped:0 overruns:0 frame:0
      TX packets:158501574 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:5000
      RX bytes:12508573129969 (11.3 TiB)  TX bytes:220148785267 (205.0 GiB)

In your example above, that network interface has received much more data than it has transmitted.

If the main traffic happening here is other hosts sending logs to it, it sounds expected that it receives more than it transmits.