When viewing the difference between two versions of an article, often small differences are shown as big.
For example when adding a newline into a paragraph the diff viewer is confused and it looks like a big change.
Another example is if changing something little in a paragraph and added a new one above, like a headline. Then the slightly changed paragraph is not recognised and it looks like a big change. One example http://en.wikipedia.org/w/index.php?title=Literal_Video_Version&diff=prev&oldid=293461052
Bug 5072 (3 years old) is related.