Is it a really good design and its safer to go with the complexity of multiple engines rather than bigger, more complex engines? Is there a redundancy advantage; a single engine failure will not abort a mission? Or perhaps its easier to refurbish and re-use an engine if its small and simple?

I also remember in the Ley/von Braun designs of the 1950s multiple small engines (on each stage!) were often featured. I always suspected that it made sense spending a lot of effort developing a really good design and mass producing it so it could be assembled in different clusters for different applications. The alternative, developing different optimized engines for different applications might be, in the long run, more expensive and more trouble-prone.

I've always been interested in the meta-engineering aspects of a design philosophy. There is always a happy medium between optimal design solutions and the unnecessary complexity of too many solutions. Robustness and efficiency are often contradictory; too much of one detracts from the other.

Or as a competitive racer might put it, "If it won't break, its too heavy."