OK I ran a test for you. Recorded a video, actually two (one landscape and one portrait) on an iPhone and sent both via text messaging to my Note 5. In each case (on At&t) the video was compressed to 3gp. Both lost considerable resolution and played back in small windows. Still images, while they do get compressed over mms, will display in fairly good quality and in full screen or according to the format in which they were taken. I never send video over standard mms and will share them via a cloud drive service such as Dropbox or Google drive. By doing so, you can share full resolution and don't have other limitations imposed through messaging restrictions. iOS devices get around these limitations using apple servers and iMessage.