Share your experience!
How did you transferred the videos from your other phone to your Z3?
could you take a screenshot and post it here or upload it to box or dropbox and post the link?
screenshot = press and hold volume up & power at the same time.
You can use a small app to do so
unlock the phone > tap the Square button > tap on the pink icon
It seems that your PC it's messing the videos, are the videos recorded with your S3 or were MMS?
Recording the screen will create small videos, that's normal. as for the transferred videos that's quite odd, what if you download a different video app and test them.