If you want to send a video file as an MMS on your Samsung Galaxy S5, then it may happen that the following error message appears on the display:
"File can not be attached. The video file is too large. The size will be changed"
You can either cancel the process or crop the video. If you select "Crop", a small video editor opens on your Samsung Galaxy S5. Here the length of the video can be varied. Unfortunately, you can only send a small portion of the video via MMS. You have to cut the video and the video will also be compressed.
The background is that MMS data is limited from the mobile operators. This is limitation is set to just a bit over one megabyte. As most of videos exceeding this file size by far, it is not possible to send them as MMS without compressing and cropping. Therefore your Samsung Galaxy S5 offers you the option of cropping.
If you wish to crop your video more professional, then there is a video editor from the Samsung Galaxy App Store available. With this editor the video can be better changed to a compatible format for MMS.
You know now why the video can not be sent via MMS from your Samsung Galaxy S5 in full size and length.