Can somewhat confirm. Unfortunately with iOS this is the default format. I’m not sure if the forum software also performs transcoding. I’ve seen video sizes increase drastically after an upload, if it works at all.
That's an interesting possibility (that the video is transcoded on upload).
I spent some time poking around the XenForo forums and it seems like many people run into this issue in various ways. The core problem in most cases stems from XF having multiple places on the backend to regulate attachments and uploads depending on what/where/how it's being uploaded, and usually the admin has forgotten to change one of the many settings.
Since I have no access to the forum backend here, we'll just assume it's set up correctly, which brings us to the other possibility - transcoding. I couldn't find any documentation/evidence that this happens for videos except in the "Media Gallery" extension (which I don't know if we're using but I don't think applies to Attachements anyway.) BUT, it was clearly stated to happen for images, which can result in much larger filesizes that move an acceptable file into an error.
Now, since the forum doesn't recognize .avi as a video file, but allows it to be uploaded, it's possible that it's either 1. trying to recompress or resave it as if it were an image, or 2. applying rules to it that should not normally apply (e.g. image max pixel setting) and encountering some kind of error that it interprets as "file too large."
I'd have to see backend logs to see what's happening for sure but I guess for now maybe I'll just try re-encoding it here to mp4 or something.