I'm using my data as internet. Here's how much data it takes to post on CE:

That's a little too high. Let me try. Here's the stats after restarting my firewall:
Starting Upload: 2.82 MB
Starting Download: 63.90 MB

Edit1: here's the stats after posting the above paragraph. Accessing CE only, no other internet connection.
Upload: 2.83 MB
Download: 63.91 MB

Edit2: Uploading picture below. Size of picture: 551,320 bytes
https://gamefaqs.gamespot.com/a/forum/a/abc39d11.jpg

Edit3: here's the final stats after posting everything above this line. Accessing CE only, no other internet connection.
Upload: 3.58 MB
Download: 64.01 MB

Total data used to upload/edit a single post and upload 551,320 bytes worth of picture:
Upload: 3.58 - 2.82 = 0.76 MB
Download: 64.01 - 63.90 = 0.11 MB

I dunno but everything checks out on my end TC. @Naysaspace