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Make sure the aspect ratio isn't simply in the player you are viewing the video file with.
In fact there is no reason for the file itself to change, aspect ratio or otherwise! Test this by copying the file back from the Samsung to a computer, and see what the aspect ratio is when viewed on the computer.
In fact whatever app you are playing the video with, may have an option or setting to readjust aspect ratio to fit the screen.
I agree, the program that manages the transfer is somehow, inappropriately, changing actual aspect ratio in the file, in all likelihood.
I think that Aspect Ratio might be either an inherent aspect of the program (i.e. it can only 'handle' certain aspect ratios, a limited choice that does not match the aspect ratio natively 'seen' or produced by your mobile camera), or a Setting ... and/or both.
You might want, quite simply, to use a different program for camera transfers. Or, simply transfer videos by hand as it were, by selecting the video on your mobile device, then copying it to e.g. the Cloud, and then posting from the Cloud storage to your Twitter/FB accounts.
I have Opted Out, not knowing much about Playmemories Mobile settings.
I still suggest using something different, not that program/app, for transfers of files.
Are you still interested in an answer?