videos downloaded from phone occasionally grainy bad quality?

recently ive been recording videos on my phone then emailing them to my PC (lenovo ideapad 100), but sometimes when i watch them, they become very bad quality, and that video will stay bad quality every time i watch it. on the other hand, some videos have perfect graphics, and some have not bad... it just depends on the video. How can i stop this from happening? is the video being simplified on email so it fits or something, cos the shorter ones do seem more likely to be better quality, from what ive seen. thankyou!