Disclaimer: this solution only works for downsampliing, if you know how to enhance the video, please share it with us
Hi iphoner, unfortunately, you need to record it using a higher resolution device and use Handbrake to convert your videos.
I used iPhone 6+ to record videos using Quicktime and ran into the same problem.
To download video for iPhone 6+, you need to download video with this size of 1920x1080.

1) Change "Anamorphic" to "custom"
2) cancel "keep aspect ratio"
3) Set the size (width and height) that you want (warning u can only reduce it / you need high resolution as the main video)
Other formats acceptable for the device:
ipad: 1200x900, 900x1200
iPhone 5.5inch / iPhone 3.5inch: 1920x1080,1080x1920
iPhone 4.7inch: 1334x750, 750x1334
A more detailed guide can be found in this Apple document.
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