You came from imessage, and you now have to see the sorry state that is android messaging. The basic text app is just that, a basic sms app, no different than on a basic phone, better interface and keyboard but that is it. So when you send a picture message it gets downgraded to junk quality. There are other, better options like whatsapp, hugely popular and works like imessage, just no sms downgrade. However, getting those lazy iphone users to switch apps can be like pulling claws our of a grizzly. In the near future there will be some drastic changes. Do a search on RMS. To much info to type it all.
One option to share full sized vids to imessage is to use the sharing app built into your phone. When you take a pic or vid, hit the share button and it will give you the option to use link sharing, then it will let you choose which app. It will send a link they can view in full resolution. I freely admit it is not nearly as nice as just using the built in sharing inside of your messanger, nor will it show the video thumbnail inline in the messenger. But it does work, and it only takes a few seconds longer. This, however, is not nesessary if using Whatsapp or telegram, allo, line, kakao, wechat, ect. Just for the stock sms app.