Xiao and McEnery (2005:70) compared two reference corpora, the 100-million-word British National Corpus and the one million-word Freiburg-LOB Corpus, and achieved almost identical key word lists, thus concluding that the size of the reference corpus is not very important in making a key word list.
wendy and Corbett (2009) point out that "key words are more sensitive to the size of a corpus, for example, longer texts will contain fewer unique occurrences of particular words".