Zend Framework Composer Packages

I would like to add dependency to zendframework/zend-db package, so I added it to my composer.json:

"repositories": [
    {
        "type": "composer",
        "url": "http://packages.zendframework.com/"
    }
],
"require": {
    "php": ">=5.3.2",

    "symfony/class-loader":  "dev-master",
    "symfony/console":       "dev-master",
    "symfony/filesystem":    "dev-master",
    "symfony/finder":        "dev-master",
    "symfony/locale":        "dev-master",
    "symfony/yaml":          "dev-master",
    "doctrine/dbal":         "dev-master",
    "zendframework/zend-db": "dev-master"
}

The problem is that composer installs entire zendframework/zendframework package.

Any idea why?