You need to remember that iMessage is NOT SMS/MMS. Apple's greatest hit (and biggest mistake when you switch) is to have made their messaging system 'seamless' between iMessage (similar to Facebook Messenger, Whatsapp, or Hangouts) and SMS/MMS (the actual, old messaging standard).
When sending a video on an iPhone, if the recipient is also using iMessage, the video will be sent through the iMessage system, not MMS. But if you send the video to a non-iMessage user, then it gets compressed to fit the MMS standards. Android sends everything through SMS (text only) and MMS (multimedia) because their "i-Message-like" system, Hangouts, is used as a separate service. As such, you need to compress the video's size, both in file size and in resolution.