Gil,
You're forgetting about powerplants (The bigger the more power, and thus destructive potential and stronger shields) Crew (Each ship is going to need a trained bridge crew and Humans can't simply throw them into teeth of battle like the Fang can) and sheer mass (Harder to destroy a large object than a smaller one)
Think of it this way - Imagine a huge open plain. On one side you have a million soldiers armed with LMGs and RPGs (Fang fleet) and on the other you have 100,000 Apache gunships (Human fleet), with their 30mm cannons, minimissile launcher and missile launchers. Both have parity in terms of weaponry (Pound for pound) but the soldiers have to rely on their strengths in numbers to win the day.
All of the civilisations the Fang have so far conquered and subjugated have been smaller than them, so while the conquered civ's might have had the tactical, organisational, or even a moderate tech advantage, the sheer numbers of the Fang fleet swallowed them up (total number of fang capital ships is estimated to be in tens of billions)
Humans are the first species they've come across where swarming tactics are not immediately effective.
Aside from all this rationalisation, the idea of large ships of civilised human society breaking waves of barbaric alien ships appeals to me.
Here's a little preliminary picture I drew of the two most important Fang ship classes:
It's on the same scale as the last drawing.
Attack ships form the backbone of the Fang fleet - This mass-produced tin bucket of a ship is found anywhere in the Fang empire, from invading virgin territory to pacifying territory held for centuries.
It has 13 dual heavy laser cannons, each barrel yielding a 500Gt blast.
It also has 79 missile tubes and 12 launch bays for drop pods.
It has approximately 3m of layered titanium armour and a single particle shield.
The Stinger Ship is used in roles for which the Attack ship would be unwieldy - mainly fighter defense, star system patrols, and scouting.
It has 11 laser cannons each with a yield of 200Gts.
It only has a navigational shield and half a metre of armour.