The percentage of ethnic Germans in the territories taken from Germany after the First World War varied from place to place. In Danzig it was the highest, around 90%, in other places it was obviously lower. In the Memelland it was about 50%.Parts of West Prussia given to Poland - or the "Polish Corridor" (Pomerelia) - had a German-speaking minority of 40% according to the census of 1910 (which - however - exaggerated the percentage of German speakers, as all other censuses including the census of 1905 and the census of School Children from 1911 prove). But parts of Posen and Silesia given to Poland certainly did not have so numerous Germans.
The 40% figure refers to the territories lost to Germany as a whole, consisting of areas where the percentage of ethnic Germans was very high, as in Danzig, and areas where it was much lower, as in the rural areas of the Posen Province.
The estimate of about one million Germans leaving the lost eastern regions was made by Hermann Rauschning in his 1930 book "Die Entdeutschung Westpreussens und Posens".
For example, according to the 1910 census, of the 2.1 million population of the Posen Province, a bit over 800,000 were German-speakers, constituting 38.5 %. That figure included German officials and troops stationed in the province, but they would have accounted for only a small proportion of the total; it must be assumed that the great majority were genuine residents of the province.
After the First World War, the percentage of ethnic Germans in the part of the former Posen Province that was given to Poland, ie by far the greater part of it, fell to only 7 percent. Part of the drop was due to some border regions of the Posen Province with mainly German populations being left with Germany, but that cannot account for most of the drop, since a large part of the ethnic German population had lived in the city of Posen and other major towns, which were given to Poland. The cities in the western part of Poland lost about 85% of their German inhabitants; the number of Germans in Posen city fell from 87,000 in 1910 to only 11,000 in 1931.
Other sources that I have seen state that 600,000 ethnic Germans left Poland after the First World War (which might include persons who left the former Russian and Austrian zones). Whatever the figure was, it was substantial. Some of the emigration was voluntary, but there can be no doubt that a certain amount of coercion was used, since it was the policy of the immediate post-war Polish Government, dominated by Dmowski's National Democrats, to push as many ethnic Germans out of Poland as possible.