If your message contains only emoji, they should appear larger.
From Slack's help pages:
Update May 2017: This is now live on Signal-Android & Signal-Desktop.
I had the same idea and got pretty deep into the weeds of JSQMessagesViewController, was going to open an issue but found this recent issue unfortunately.
Sounds like we need a "Can'tFix" or an "Upstream Issue" tag.
Now that JSQMessagesViewController is being removed, I think this request can be revisited to bring feature parity to iOS.
This feature is complete and will ship in v2.19.
Most helpful comment
This feature is complete and will ship in v2.19.