Sometimes iMessage reduced the picture quality and the size of the sended picture

All I can provide is what is in the iOS 12 manual at the link I gave you:

“Send photos, videos, and audio messages on iPhone

In the Messages app, you can send photos, videos, and audio messages using the iMessage or SMS/MMS service. You can share, save, or print an attachment. The Photos iMessage app suggests photos to share based on who you’re messaging with, what you’re talking about, and where you’ve taken photos together. Your carrier determines the size limit of attachments; iPhone may compress photo and video attachments when necessary.”

And while your iMessages are encrypted, that doesn’t mean the carrier’s system cannot refuse transmission and thus force the service to compress attachments before sending.

In my own experience sending hi-resolution large file size native pictures as a message attachment in iMessage over cellular is inherently variable. That’s why I just use email and wait until I have a WiFi connection to send pictures like that (but even using iMessage over WiFi works far better than over cellular, in my experience).

If you’re sending as iMessage over WiFi, it should not compress. If you’re sending as iMessage over cellular data then depending on your carrier, yes, images may be compressed.

If sending as MMS texts then it is up to your carrier and their system setting as to if or by how much they compress images.

Welcome - Apple Support

iOS and the iOS messaging app does nothing to those images - it sends them as they are. So whatever is happening is happening as a function of whichever data network you’re sending them with.

For what it’s worth, I always use email and WiFi to send photographs as it is much more reliable in delivering the image at the resolution I pick rather than dealing with the vagaries of cellular data networks.