I found a Video that claims this is how the European borders looked like at the time. Germany is quite a bit bigger than what I had expected.
If there was a German invasion into Poland, Ukraine and the Baltics just before the first World War, why isn't it something that is talked more about?
It isn't talked about, because it didn't happen. You pointed out much of Eastern Europe is in Germany's colour, but there is also Germany in Belgium and eastern France, and Austria-Hungary occupies Serbia and parts of Romania. Especially this last thing shows: the map is from 1917, on the height of WW1.
I think this is a typo, and meant to be 1917, demonstrating the maximum extent of German occupation in Eastern Europe following Russia's withdrawal from the war, as you can see from this map from the West Point atlas.
Similarly, it shows the occupation of Belgium and parts of France around the same time, as in this map from the same source.
The 1907 borders of Germany were the black lines in the map you linked to.
