That's a very common problem with video content when transferred via text messaging. The problem with that relates to MMS and with how different carriers interact with each other.
When you were using an iPhone, with iMessage all text messaging, and any attachments, get transferred through Apple's iCloud server farm, no matter if your carrier was AT&T, or Verizon, or Mint Mobile, etc. Everything goes through iCloud. But with Android, things like video content often needs to get scaled down when it involves media content (photos and videos). SMS and MMS are dated, very low bandwidth communication standards, so when it involves higher res content (their file size is also larger accordingly) media content will often get scaled down on-the-fly as it gets transferred from one carrier's servers to the other. There is no working, established Android equivalent to iMessage currently. Google is touting RCS as an iMessage equivalent for Android, but note that Apple has no intention to include RCS support in its iMessage app so it's still a matter where SMS/MMS are the primary text messaging links between the two platforms. A future scenario might be iMessage will remain as the standard for Apple and RCS will become the standard for Android, but the two platforms are still more or less isolated from each other.
There's also the problem in that the different carriers have set their own arbitrary file size limitations to text messaging attachments so that just adds to the problem (they have been increasing them lately because so many people are upset about this). So depending on your carrier and the text messaging app you use, video content will be scaled down differently.
On the receiving end for you, it's not a matter of some setting you can adjust fix this resolution issue (allowing for some options that some text messaging apps do have), we all have to just wait for the proprietary pissing matches to subside. On the sending end, if people on the receiving end of your text messages have issue with photos or videos you sent them, you can instead use sharable links that most online file storage services provide (Dropbox, Google Drive, SpiderOak, MS OneDrive, etc.); or you can post them to file sharing services like Firefox Send (works with any browser) and just send the provided link.
Or if you still have your iCloud account, you can upload things using a web browser and share your content from there.
https://www.icloud.com/