Different size of indexes between MongoDB and PostgreSQL

Question

I have a dataset at about 11 GB that contains some attributes as shown below:

POS_ID,STATUS,LON,LAT,TIMESTAMP
116351,8,157.0625,-24.36573,2013-12-31 17:03:00.000
116357,0,148.9484,-20.34638,2013-12-31 17:03:00.000
116358,0,152.1151,-11.07703,2013-12-31 17:03:00.000

I have used two different databases for storing the data. More specifically:

MongoDB

I convert this dataset in GeoJSON format and then I index (BTree) the attribute "POS_ID".

PostgreSQL

I store the dataset in CSV format and then again I index (BTree) the attribute "POS_ID".

I observe that the index size is significantly different between the two systems. In case of MongoDB the index size is 6 GB and in PostgreSQL is 3,1 GB.

If I index the "TIMESTAMP" attribute the index size is the same for both dbs.

Does anyone knows what is the reason that in MongoDB the size of the index in almost 2 times bigger? And in the second case, what is the reason that the indexes have the same size?