“Show Full Message” is an important feature.
In my opinion, the text is a bit visually disturbing in chats with rather short messages.
In such a case, the “Show Full Message” button is as long or even longer than the actual message.
Because of the size ratio to short message texts, an icon would probably not be better.
Maybe it would make sense to integrate this feature into the context menu?
Then you would need a tip or click more. However, you don’t need the function all the time.
But the chat history would look tidier.