Difference between totalSize() and dataSize()

Guys i am running this command in terminal because i want to find out the collection size(data+indexes).The problem is that the command db.mongodbbucketright2index.dataSize() returns 368274259 bytes and db.mongodbbucketright2index.totalSize() returns 74448896 bytes.Why is this happening .I know that totalSize gives me the data size + indexes size but whats up with datasize and why it returns so many bytes?