Here's the comment:
// level-0 file size of 420,000,000 applies to output files of this level
//   being written to level-1.  The value is 7 times the default maximum
//   write buffer size of 60,000,000.  Why seven times:  6 level-0 files typically compact
//   to one level-1 file and are each slightly larger than 60,000,000.

Which makes perfect sense, I thought it was another use of the magic "42" number rather than a real calculation.  Guess I was wrong!