I was uploading a short video on instagram with an iphone 15 that was incredibly pixelated upon final the upload. I uploaded the same video on an iphone xr and the final upload quality was much better.

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Now, I have done all regular tips: set iphone at most compatible, film in 1080p, upload at high internet speed, etc etc.

All things being equal I can upload a nice HD video on the iphone xr, but the iphone 15 hyper pixelates the video.

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Any tips to sort this out?