If you check the Warn me when the number of words in the translation is... checkbox, the number of words will be checked according to the settings made below. You can set and activate a lower tolerance limit and an upper tolerance limit. The default values are 50% for the lower limit and 200% for the upper limit. This leaves plenty of room for differences between languages as regards to the number of words used.
However, even if the translation is beyond theses limits, there is no guarantee that the translation is wrong - and vice versa! Always keep in mind that you are the one to do the final checking!