defensive fleets

I've been reading everything I can on defense in preparation. These are some 'golden rules' writen by someone who seems to know his stuff (ie knows all the formulas inside out)

His golden rules for defensive fleets are :-

No unshielded units other than fighters.
No ion units.
No carriers.
No more than one cruiser for every hundred fighters.
No heavy cruisers.
No more than one battleship for every ten cruisers.
No ships bigger than a dreadnought.
The total armour value of the fleet may not exceed the total armour value of the defensive structures


So a typical defensive fleet might be :-

5000 fighters
50 cruisers
5 battleships

There is one rule I don't understand though...
The total armour value of the fleet may not exceed the total armour value of the defensive structures"

Seems to me if you follow this rule you end up with very small defensive fleets. I know you don't want huge fleets that allow profit from the debris - but I'm trying to figure the optimum size.

For example atm with say 3 photon turrets which eqautes to say 72 armor that only allows a tiny defensive flleet.

With a 2-pring p-shield combo that allows for 12000+ armor but it still means a smaller defensive fleet than other guidelines I've seen.