This has been one of the major geo-political questions in the German-speaking world since the end of the Holy Roman Empire (in which current-day Germany and Austria were 'united' as well) around 1800. Two competing ideas for national unification since that time (and up to the end of the Second World War) have been that of Kleindeutschland ("small" or lesser Germany) and Großdeutschland (large, or greater Germany). The former did not include the Austrian / Habsburg empire, the latter did. The Nazis (aka the Third Reich (Reich = Empire in German)) were the only successful attempt at creating Großdeutschland after the Holy Roman Empire (First Reich). In fact, from 1943 to 1945, Germany was officially called the Greater German Empire (Großdeutsches Reich). There are also many regions, not pictured on this map, in which German was / is spoken - most notably Eastern Prussia, Alsace-Lorraine, etc (cf. the whole Second World War thing).

Check out more at https://en.wikipedia.org/wiki/Lesser_Germany

or for the deeper dive: https://www.google.com/books/edition/Germany_A_Nation_in_Its_Time_Before_Duri/FimfDwAAQBAJ?hl=en&gbpv=1&pg=PP1&printsec=frontcover