O.k., for relatively new users of Slack like me, you definitely can get a wider version of a thread, but you can’t expand it directly from the thread in the righthand pane (I don’t think).
What you have to do is click on All Threads in the lefthand menu pane. If you had channel content in the central message area before, it will be replaced with a list of discussions that include both channels and threads. Just scroll until you find the thread you want and then you can view it in a wider format and also continue the conversation in less cramped fashion.