MongoDB unique index size more than document total size

Question

I am using MongoDB Atlas.

I recently found out that the total size of one of my collection is 67.75 mb, but one of its unique index is taking 294 mb, how is this even possible? How can the index size be more than double of the total document size?

Please take a look into the screenshot below

Mongodb Indexes Screenshot

Answer 1

What you see in the picture as 67.75MB is actually the storageSize which is usually the compressed size of your collection (db.myCollection.stats(1024*1024).storageSize ) , to view your actual collection size in MB you can execute:

 db.myCollection.stats(1024*1024).size

This will show your collection uncompressed size which is expected to be much more then only 67.75MB ...