Which app are you actually using? I'm guessing that you are using Google Messages & that's why you see the difference in size, as there's additional data in the app you are using, ie: SMS & MMS messages, contacts etc, which if you switch to Samsung's Messaging would populate & increase it's size on your device.
The apps themselves are:
Samsung Messages is ~ 50 Meg
Google Messages is ~ 90 Meg
Google Messages appears to be better suited for RCS than Samsung's & that could explain the difference in size of the app itself.
Source for the size of the actual apk is apk mirror. Note that this is the full size of the apk to download & Install.
I'm using samsung messages, disabled Google app. I suspect additional samsung app resources required for messaging app tonwork are spread across other services. But still 200MB for sms/mms installed seems large. Oh and I only have 3 sms messages:)
