EC2 used space increases after resizing volume

I have followed AWS guide to expand the 8GB volume to 16GB.

What I have done is:

1. Take snapshot of 8GB volume
2. Create new 16GB volume by the snapshot
3. Detach 8GB volume from EC2 instance, then attach 16GB volume to that EC2 instance

After that, df -h:

Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       16G   11G  5.3G  67% /

Why the Used size increase to 11G? It should be 8G.