Basically I could name pretty much any book now to compare the page count, but the following example really shows you just how big the difference can be. I bought the Commonwealth Saga by Peter F. Hamilton in the German language - four books all in all. In English you get two books though. Confused yet?
The English book Pandora's Star is 1152 pages long.
The German translation, which present itself in two parts, adds up to 1456 pages.
Even if you consider that maybe the print of the English version might be (a lot) smaller this is quite the difference and it also explains why they decided to split the book in German - I'm not going into detail as to what this means when it comes to reading the books, that's a whole other story, obviously ... let's just say it's probably a good idea to start with the first part and not "in the middle" of the book, so to say.
How about another example?
Harry Potter And The Philosopher's Stone has got a measly 224 pages in English and 336 in German. Magic? You could almost get that impression.
So what is the exact percentage regarding those differences in page numbers or, more precisely, word counts? That really depends on the kind of text and the languages in which a book gets translated. Generally you may say that a translated text will be 10-20% longer than the English version (that difference will reflect in the actual word count, not so much in the page count which may vary due to font size and formatting).
The interesting question would be, how much of this is the fault of the translator (who's got a thing for writing/translating in a rather florid and wordily style) and how much has simply got to do with the fact that you often cannot translate one word into another without loosing or changing the meaning, thus making it necessary to translate one word with a whole phrase to convey what the author wants to say. I think it's mostly the latter which is to blame.
Back to the examples above.
The first one would be 26% longer, and the second 50% ... this is way more than the above mentioned percentage. Maybe they did blow up the font size to make the difference even bigger, who knows? Where Harry is concerned, there probably must be magic involved. Even in translation.
Have you ever noticed the difference in page and/or word counts of translated books? Let me know.