The DAX is made of the 40 most valuable German companies. That’s how it is defined. So the companies not in it, again by definition, matter less.
Not to be too nitpicky here but these are only the publicly traded companies. You have a number of pretty large German companies that are still entirely private such as Aldi, Schwarz Group, Boehringer or Bosch.
https://www.famcap.com/top-500-german-family-businesses-the-...
Not all of those are listed, or listed in Frankfurt