why my mongodb fileSize is much bigger than storageSize in db.stats()?

I have a db named log_test1, with only 1 capped collection logs. The max size of capped collection is 512M. After I inserted 200k data, I found the disk usage of the db is 1.6G. With db.stats(), I can see the storageSize is 512M, correct, but my actual fileSize is 1.6G, why did this happen? How can I control the disk size is just my capped collection size plus index size?

> use log_test1
switched to db log_test1
> db.stats()
{
    "db" : "log_test1",
    "collections" : 3,
    "objects" : 200018,
    "avgObjSize" : 615.8577328040476,
    "dataSize" : 123182632,
    "storageSize" : 512008192,
    "numExtents" : 3,
    "indexes" : 8,
    "indexSize" : 71907920,
    "fileSize" : 1610612736,
    "nsSizeMB" : 16,
    "dataFileVersion" : {
        "major" : 4,
        "minor" : 5
    },
    "ok" : 1
}