You are confusing two things. I am talking about the files making up a database. MongoDB doesn't use one file per database - instead it allocates files up to 2GB in size (the first one starts at 64MB and then they double in size until the 2GB limit). So for example a database that is 200GB will consist of (roughly) 100 2GB files.

What you linked to is a consequence of memory mapping. Mapping a single 2GB file in a 32 bit process will use up virtually all the address space and you couldn't map more than one at a time.