dd question, what am I doing wrong?

[Daniel B. Thurman]

I am copying a disk/partition to another disk/partition
using dd, with the following:

BEFORE:
Disk1: partition#1: psize=100G, size=97.65G, used=91.23G, unused=6.42G
Disk2: partition#1: psize=250G, size=244.14G (newly formatted)

I did a dd if=/dev/sda1 of=/dev/sdb1

AFTER:
Disk1: partition#1: psize=100G, size=97.65G, used=91.23G, unused=6.42G
Disk2: partition#1: psize=250G, size=244.14G, used=237.72G, unused=6.42G

WTF...

Then I:
1) mount /dev/sdb1 /mnt
2) cd /mnt/b
3) df -h .
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb1              97G   90G  1.6G  99% /mnt/b
4) umount /mnt

5) mount /dev/sdb1 /mnt
6) cd /mnt
7) df -h .
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              97G   90G  1.6G  99% /mnt/b
8) Umount /mnt

Hmm... this is not what I expected... I had assumed that somehow
the free disk space would somehow be reclaimed...

What am I doing wrong here and how do I get around this?