[Suicide Junkie]

A rule of thumb is not to let your tech for hull size fall more than 1 behind your enemy. Now that's just a rule of thumb, you could beat them with even smaller hulls...

As an example, I have fought off Druk WMG Dreadnoughts with hordes of missile frigates.
The key there was a chokepoint between us, and my stack of 150 frigates.
Not to mention being on the correct side of the warppoint so the fleets started at a safe range for me, and the choice attacks to pick off vulnerable single dreads.

The delaying action by the frigates gave me enough time to upgrade my economy and research more tech, leading to eventual victory.