Growth depends on many things. Your approach should include three things; offers, subscription forms and social media. First, in exchange for an email address, offer something (eBook, download, trial offer, etc.) free. Second, make it easy to subscribe. Popup forms may be annoying, but stats show they work. Lastly, use social media to connect with others. Direct them to your content offerings, mentioning the offers available. It'll take time, but you'll get more qualified subscribers.
NOTE: Some people opt to purchase lists. Doing this is hit or miss. You may increase your database, but looking at Open Rates and Click-thrus, you'll see a lack of interaction. They don't know you and will probably unsubscribe or mark you as SPAM.