I mentioned in the Flash DROD translation thread that written English tends to be more compact than a typical European language. Here is a citation of sorts for this claim that has figures that IBM has collected.
http://www.w3.org/International/articles/article-text-size
The above page cites this page.
http://www-01.ibm.com/software/globalization/guidelines/a3.html
The short version is that sometimes short texts, such as you might use as a heading or a label in a graphical user interface, can expand to even three times the size of an English phrase in some languages. Typically longer texts expand less in relation to the original as compared to shorter texts.
You might expect some written languages, notably Chinese, to actually be shorter than English on average. But generally the problem is that if a text grows longer, it messes with the originally planned layout in surprising ways, and you want to plan ahead for that situation.
This is probably useful to keep in mind when planning translations and localization. So I figured it deserved its own post here too. Just in case anybody was thinking of planting a big friendly wikipedia-like
citation needed on my claims... I have one right here