arc diff uploads more than just the literal diff, if I understand correctly

in complex production environments (ours: LB out of our direct control -> nginx -> nginx -> fpm -> phabricator), when slightly bigger diffs fail to upload with nginx 413 Request Entity Too Large it's very difficult to figure out if the developer hitting this issue is doing something profoundly dumb or if there's a real misconfiguration leading to diffs of reasonable size being too big to upload.

ideally this would test uploading whatever is considered a reasonable maximum , and on failure fall back to smaller sizes and report "succeeded in uploading 14MB diff, failed at 18MB", but anything that can tell me roughly at what point things start breaking would be great.