Southend will likely have Class D very soon (they have applied for it already), even though they had 70,000 fewer passengers in 2012 than DSA.
Don't forget that Southend's easyJet services only got underway in April 2012 so Southend will overtake DSA over a full operational year. Southend has the advantage of a direct rail line into London within 55 minutes and its on course for a million pax per year for 2014, so its application for controlled airspace should be successful.