Cannot delete admin database

Hello guys,
my admin database is too big, so I can’t connect to my mongodb.

When I try to connect, it says: Error: BSONObj size: 16793989 (0x1004185) is invalid. Size must be between 0 and 16793600(16MB) First element: _id: “admin.username”

I tried to delete admin database with this command, but I can’t do even that:
mongo -u “username” -p password --eval “db=db.getSiblingDB(‘admin’);db.dropDatabase();”

Shows the same error: Error: BSONObj size: 16793989 (0x1004185) is invalid. Size must be between 0 and 16793600(16MB) First element: _id: “admin.username”

I do not know what to do?
Please help.

Zeljko