The Spanish Tree has 63 skills and the German Tree has 124. Why is German Tree twice as long?
Because not all Duolingo courses are created equal. They are created independently by separate teams.
For example, the Ukrainian course is one of the shortest on the website, while the Norwegian course is by far the longest. I believe there are three times more words in the Norwegian course (around 3200) compared to the Ukrainian course (around 1000). "Joke" languages like Klingon and High Valyrian generally have even less.
You can google "length of Duolingo courses." Someone did their research and made a list of courses by number of words, skills, subjects etc. though that data may be out of date.