Network Inspector in Dev Tools is truncating large requests. Example: go to espn.com and inspect network traffic. Look for a large script (i.e. espn-defer.js). Notice the response truncated in Network Response Inspector.
Yes, currently responses over 1 MB are truncated this is done to avoid hogging memory if you don’t need the response data.
In any case, I agree we should offer some control over this. We have bug 1159078 filed for this issue.