The issue you're having is a long-standing problem involving transfers of photos and videos as text message attachments. It's not just an Android vs Apple thing, nor is is it directly a RCS vs iMessage matter. Back when email was the more popular way to exchange attached media files, it was only a minor issue when it came to really sizable attachments (higher res files). So 20 MB files were easily transferred, and it made no difference what operating system was on either end (sender or receiver), nor was which email service (i.e. Gmai to Yahoo to Hotmail to whatever). Email protocols were standardized as an industry standard.
But now text messaging is the norm, and it's full of incompatibilities from the start and continues even now. Proprietary standards and protocols prevent seamless interactions, and competing corporations care more about intellectual property much more than they care about their customers/user base. So text messaging, which often takes place only through cellular network exchanges, has a lot of limitations when it involves interactions between different carriers. File attachments are limited to under 1MB with a lot of carriers. So this problem has been around before RCS even came into existence. And that's a big factor in your problem with high res files being automatically scaled down to low res files in transit. As for RCS, Google has been really pushing that to be the text message standard protocol, essentially replacing SMS and MMS, the two protocols that remain as the only standards that all text messaging services support. The problem being SMS and MMS are very, very dated protocols and both have only basic capacity in today's technology. RCS is a modern protocol that is actively developed and supported but currently limited only to the Android platform as Apple has so far refused to support RCS in its iMessage app. So the benefits to RCS are only in the Android side of things. Conversely, Apple's iMessage protocol is also actively developed and well supported but it is limited only to Apple owners. Apple refuses to allow any other platform to use its iMessage standard.
When it comes to a RCS to RCS exchange, file size limits aren't an issue, but keep in mind not every Android text messaging service includes RCS support and not every cellular service support it.. When it comes to iMessage to iMessage exchanges then file size limits aren't a significant issue either. Even though RCS is not Open Source, Google does make its api open for any platform to use. Apple opts not to. So the bottom line falls back to the MMS and it's limited abilities to handle current technology. It's still the most common way for all the different cellular services to exchange file attachments.
If you want to send high res to others, copy it into an online file storage service (Google Drive, MS OneDrive, Dropbox, SpiderOak, etc.), obtain the sharable link for that file, or for a folder with multiple files in it, and just include that link in your text message. Your recipient can view or download the high res content using the link.
