def get_shrink_type(plugins):
    """Gets the type of shrinking process requested by the user, taking into account backward compatibility
    values

    :param dict plugins: the part of the manifest related to plugins
    :return: None (if none selected), "vmware-vdiskmanager" or "qemu-img" (tool to be used)"""
    shrink_type = plugins['minimize_size'].get('shrink')
    if shrink_type is True:
        shrink_type = 'vmware-vdiskmanager'
    elif shrink_type is False:
        shrink_type = None
    elif shrink_type == 'qemu-img-no-compression':
        shrink_type = 'qemu-img'
    return shrink_type