The Original/Default Source quality for Onlyfans is 1920x1080, with a max file size of 3gb.
In what you are referreing to a few things may play a role in this process.
-She may need to upgrade her filming protocol solely for Onlyfans.
-She may be recording some in 720p and a select few in 1080p to save on upload times.
-She may be recording them fully in 1080p but the resolution may be degraded after the upload via Onlyfans.
-Depending upon the software she uses to edit the video, the outcome may be degraded after the rendering if its not done correctly, and then degraded even further after the upload.
-As you stated the person downloading may have to select "Original" vice "720p" prior to the download, to get the "Full Resolution", on how it is perceived to be consumed.
If you look at her YouTube videos, the quality is all over the place. Some you can download in 4k, others in Full HD others only in 1080p.
Everything is inconsistent, so if I were you I would just take what you can get. We are all here for the same thing.
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