Hello!
I’ve been recently looking at my systemmonitor ethernet input and output, and they seem to be very large! I am seeing on the order of multiple GBs over the course of 4 days. Does HA really use that much data?
What values do you see in yours, and over the course of how many days?
The answers you might see to your question will vary depend on many things. HA can make use of ethernet to communicate locally with other devices (like wifi sensors etc). It can also use ethernet to talk to the internet; this is generally frowned upon in HA circles, but nonetheless accepted by many as ok when you want to make use of outside data (like weather). It could also use ethernet for a camera stream on shodan, lol!
Of course some of those things are benign, and some are not. To know if your traffic levels are ‘normal’, you need to have an idea of what kind of traffic is expected from each of those many sources I mentioned above (and the infinite other possible sources I didn’t mention).
So say if you like have a ton of esphome (wifi) power sensors (lots of frequent data), you might see appreciable ethernet traffic from those alone. GB worth? …yes it is possible depending on the scale and configs involved. Otoh, could a small stream to shodan slip into that flow of data without being noticed? Yep… at least you are doing your due diligence looking under the carpet for these kind of things… but as you can see defining normal vs not ain’t an easy task. At least it may be useful to determine how much of that traffic is local vs online… if it is all local then you sleep better at night. 
Also to consider; do you use external db (like Maria DB on NAS) or do you perform external backups (like to Google Drive). These might cause relatively large data transfers.
For troubleshooting it might also help to set up recorder to monitor traffic, so you can eventually see if there are any peaks or traffic is distributed evenly across the day…This might point to specific tasks.