.tar.gz = same size as before?

I tried to compress 5 bam files using:

tar -czvf original_bams.tar.gz *.bam

The resulting file sizes (“ll –block-size=M”) are:

8067M file1.bam
6962M file2.bam
10662M file3.bam
7794M file4.bam
7346M file5.bam
40828M original_bams.tar.gz

There’s a difference of 3MB between the archive and the sum of the sizes of the bam files. Is this expected? I know that there is CRAM (which I will turn to next) but I’m surprised to see that good old .tar.gz has 0 effect?