Mongo.1 is compressed while the others aren't. I'd advise not to gzip it since most packagers/installers do this for you. Not sure which one you prefer, but it should at least be consistent.

1. debian # file mongo*.1
   mongo.1: gzip compressed data, from Unix, last modified: Mon Jun 1 17:54:38 2009
   mongodump.1: troff or preprocessor input text
   mongoexport.1: troff or preprocessor input text
   mongofiles.1: troff or preprocessor input text
   mongoimportjson.1: troff or preprocessor input text
   mongorestore.1: troff or preprocessor input text
   mongos.1: troff or preprocessor input text