Mongo DB backup sizes
Actually, if you don't use --gzip parameter, data is at bson format what is not compressed and you can look that data with bsondump program. 
What mongodump do, is read data from database and that result is of course not compressed. 

Here is different between those two from my system

-rw-r--r-- 1 root root 61M Mar 16 06:48 data.bson
-rw-r--r-- 1 root root 5.3M Mar 16 06:48 data.bson.gz

Over 10 fold compression ration.