Page "Lossless compression" Paragraph 35

In fact, if we consider files of length N, if all files were equally probable, then for any lossless compression that reduces the size of some file, the expected length of a compressed file ( averaged over all possible files of length N ) must necessarily be greater than N. So if we know nothing about the properties of the data we are compressing, we might as well not compress it at all.

A lossless compression algorithm is useful only when we are more likely to compress certain types of files than others ; then the algorithm could be designed to compress those types of data better.