Doing a lot of very short projects can be inefficient. It seems easier to write a book review, but writing five 1,000-word reviews is probably more time than a 6 thousand-word article. Writing 10 such articles on disparate matters would be more work than a book of 10 chapters of 8,000 words each.
The advantage of shorter writing assignments, however, is that each one is less painful than a larger one would be. There is a more immediate short-term benefit in getting something done quickly. Since scholarship is a game of delayed gratification, it is nice to have a few things that pay off more immediately, if only in a fairly trivial way.
The trick, then, is to arrive at the right calculus of shorter and longer, shorter term and longer term, things that you have to write. In my case, for example, I sometimes published next to nothing in a year I was writing a book. Since my evaluation period is a calendar year, I have to make sure that something appears or is accepted each year, even if a long-term project might take several years to come out. At the same time, I can't let myself be distracted with lots of little stuff because that creates inefficiency.