The expansion rate of a translation is the percentage of additional characters and therfore additional space needed after translation. When you translate from English to French or Spanish, for example, you may look at around 20% text expansion. German may use the same amount of words or even less than English, but you still look at an expansion rate of up to 35%. The reason is that German has generally longer words plus the (in)famous compound words like Kraftfahrzeug-Haftpflichtversicherung (car insurance) or Nahrungsmittelunverträglichkeit (food intolerance).