how to change root disk volume of ec2 instance using ruby sdk

I'm trying to specify the root disk volume size on creation of an EC2 instance. I passed the option

:block_device_mappings = [
        {
          :device_name => "/dev/sda1", 
          :ebs => {
            :volume_size => 50,
          }
        }
      ]

but when I logged into the instance it didn't seem to have taken the option that I have specified above. What did I do wrong?

[root@ip-172-18-15-69 ~]# df
Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/xvda1      25677580 13857284  11558168  55% /
none             1864568        0   1864568   0% /dev/shm