Would this assumption be correct or were there other factors involved?

That was part of it, but it had more to do with the fact that these empires were in the process of collapsing/coming apart when the war ended. In this sense, the Treaties of St. Germain and Trianon with Austria and Hungary, and the Treaty of Lausanne (which replaced Sevres) were merely recognizing de facto borders, rather than deliberately breaking these entities up. I should also add that the Treaty of Versailles was the Treaty signed with Germany, not the whole peace-making process itself.

When the war ended, the Ottoman Empire was breaking-up as a result of the Arab Revolt, and the territories that it ceded to the Allied Powers in Mesopotamia and the Levant were already occupied, with subsequent Turkish resistance preventing further territorial losses. In Austria-Hungary's case, Bosnia, Croatia and Slovenia broke away in November, and petitioned to join the Kingdom of Serbia, which then comprised Serbia and Montenegro and subsequently became the Kingdom of Yugoslavia. Italian forces occupied Trento and Trieste, and so these territories as well as those that they gained on the Adriatic Coast were not technically part of the Empire when the Treaties of St. Germain and Trianon were drawn up. Bohemia and Moravia had also been granted autonomy by Emperor Karl shortly before his abdication, and these then formed the independent Czech state. Galicia had also broken away, with fighting there between the Poles and the Ukrainians, and the whole territory was ultimately de facto annexed by Poland, and then approved by St. Germain. Hungary's territorial losses stemmed from military actions by the Czechs, French, Serbs and Romanians, who invaded the Kingdom in 1919 to depose Bela Kun's Communist government. This led to the cessation of Slovakia and sub-Carpathian Ruthenia to the Czech state, which became Czechoslovakia, Transylvania, Maramures, Crisana and the Eastern Banat to Romania, and the rest of the Banat to Serbia, while the Burgenland was given to Austria following a Plebiscite.

In all these cases, these states were on the verge of breaking up or already were, and in most cases military forces of the Allies or states counted among their number were in place to occupy these territories. None of these were in place in Germany, and neither was German de-unification desirable. Germany lost Alsace-Lorraine because it's government petitioned to join France, and this was promised by the 14 points. They lost much of West Prussia and Upper Silesia due to Plebiscites, and in the latter case access for Poland to the sea was another one of the 14 points. Northern Schleswig was lost after another plebiscite, but some returned in 1922. Aside from the Allied occupation of the Rhineland, Germany's central government still maintained power and the country was largely intact. The Allies had neither the will, given Germany's economic importance to Europe, nor sufficient forces and popular support, to occupy all of and then partition Germany.

The "German Empire" wasn't actually largely intact after the First World War. The German Empire included a large number of overseas colonies in Africa and the elsewhere - all of which were handed over to the Allies as war reparations.

With regards to the German heartland in Europe, /u/DuxBelisarius has already noted the various plebiscites held in various German region.

However, a factor he forgot to mention was that the principle of Self-Determination - wherein the people living in an area got to choose their political future - which Woodrow Wilson put forward as part of his Fourteen Points peace plan. While many cynical voices were raised against Wilson's rather idealistic vision, self-determination was actually a very appealing idea among the various minorities which had been living within Germany and the former Austria-Hungary. This is why the plebiscites were held, and thus gave democratic and popular legitimacy to the borders of Germany after the war (which only became a wedge issue after the Nazis took power and started to make a big deal about the German populations living outside of Germany).