thank you all, i will try to use suggestions and see. but.. here again is my initial confusion.
i have just used my youtube downloader app to get Taylor swift, shake it off. the file is 45mb's mp4.
how is it so low for a 4 minute song/video?? (i have tried to upload the file here so can see for yourselves that it's 45mb's and plays beautifully, but there doesnt seem to be an option here)