ubuntu image with packer template: root not resizing with flavor

[john]

New to packer.

I am building a ubuntu 16.04 image with disk size as 10G. When I create a open stack flavor with a bigger root size I was assuming it will pick it from flavor and resize accordingly. However, root size is getting fixed to the size I give in template.

The same template used for centos7.7 is working fine.

{

  "builders": [

    {

      "type": "qemu",

      "boot_command": [

        "{{ user `boot_command_prefix` }}",

        "/install/vmlinuz noapic ",

        "file=/floppy/{{ user `preseed` }} ",

        "debian-installer={{ user `locale` }} auto locale={{ user `locale` }} kbd-chooser/method=us ",

        "hostname={{ user `hostname` }} ",

        "fb=false debconf/frontend=noninteractive ",

        "keyboard-configuration/modelcode=SKIP ",

        "keyboard-configuration/layout=USA ",

        "keyboard-configuration/variant=USA console-setup/ask_detect=false ",

        "passwd/user-fullname={{ user `ssh_fullname` }} ",

        "passwd/user-password={{ user `ssh_password` }} ",

        "passwd/user-password-again={{ user `ssh_password` }} ",

        "passwd/username={{ user `ssh_username` }} ",

        "initrd=/install/initrd.gz -- <enter>"

      ],

      "disk_size": "{{ user `disk_size` }}",

      "floppy_files": [

        "http/{{ user `preseed` }}"

      ],

      "headless": "{{ user `headless` }}",

      "http_directory": "http",

      "iso_checksum": "{{ user `iso_checksum` }}",

      "iso_checksum_type": "{{ user `iso_checksum_type` }}",

      "iso_urls": [

        "{{ user `iso_url` }}"

      ],

      "output_directory": "/data/tmp/packer/output/Ubuntu-16.04-minimal",

      "shutdown_command": "echo '{{ user `ssh_password` }}'|sudo -S shutdown -P now",

      "ssh_password": "{{ user `ssh_password` }}",

      "ssh_username": "{{ user `ssh_username` }}",

      "ssh_wait_timeout": "10000s",

      "accelerator": "kvm",

      "vm_name": "{{ user `vm_name` }}",

      "qemuargs": [

          ["-nographic" ]       

      ]

    }

  ],

  "provisioners": [

    {

      "environment_vars": [

        "UPDATE={{user `update`}}",

        "INSTALL_DEV_PACKAGES={{user `install_dev_packages`}}",

        "INSTALL_VAGRANT_KEY={{user `install_ubuntu_key`}}",

        "SSH_USERNAME={{user `ssh_username`}}",

        "SSH_PASSWORD={{user `ssh_password`}}",

        "http_proxy={{user `http_proxy`}}",

        "https_proxy={{user `https_proxy`}}",

        "ftp_proxy={{user `ftp_proxy`}}",

        "rsync_proxy={{user `rsync_proxy`}}",

        "no_proxy={{user `no_proxy`}}"

      ],

      "execute_command": "echo '{{ user `ssh_password` }}' | {{.Vars}} sudo -E -S bash '{{.Path}}'",

      "scripts": [

        "scripts/base.sh",

        "scripts/cloud-init.sh",

        "scripts/wmt-release.sh",

        "scripts/ntp.sh",

        "scripts/serial-console.sh",

        "scripts/cleanup.sh",

        "scripts/zerodisk.sh"

       ],

      "type": "shell"

    }

  ],

  "variables": {

    "boot_command_prefix": "<enter><wait><f6><esc><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs><bs>",

    "cleanup_pause": "",

    "cpus": "1",

    "custom_script": ".",

    "desktop": "false",

    "disk_size": "40960",

    "ftp_proxy": "{{env `ftp_proxy`}}",

    "headless": "false",

    "http_proxy": "{{env `http_proxy`}}",

    "https_proxy": "{{env `https_proxy`}}",

    "install_ubuntu_key": "true",

    "install_dev_packages": "false",

    "iso_checksum": "sum",

    "iso_checksum_type": "sha1",

    "iso_name": "ubuntu-16.04-server-amd64.iso",

    "iso_url": "url",

    "locale": "en_US",

    "memory": "512",

    "no_proxy": "{{env `no_proxy`}}",

    "preseed" : "preseed.cfg",

    "rsync_proxy": "{{env `rsync_proxy`}}",

    "hostname": "dsdsd",

    "ssh_fullname": "dsds",

    "ssh_password": "dsds",

    "ssh_username": "dsd",

    "update": "true",

    "ubuntufile_template": "",

    "vm_name": "packer-qemu"

  }

}

Where does the resize configuration exist. Please help