Light starships do not tripple your fleet size, they multiply it by 5 (ie 5 times more damage)
But Infiltrators and ac's are not part of a fleet ever (They both are entirely useless in a flow moving fleet), fighters, bombers, cruisers, more long range, more hard hitters, no melee ships is what makes a good strike fleet. When you munition boost your fleet you should have about 800-900 combat ready MK1 and MK2 and maybe 1 class MK3 (usually best to go cruisers for that) then you can take down pretty much ANY planet, even if it has 1000 MK3 ships on it (clearing the wormhole is then of course, the problem of autobombs/missiles)
What the nice forum geeks here described is the so called "fire sale" tactic, you destroy the enemy command station and retreat for the counter attack, once its over you take out the ion cannon and mop up the remaining guard posts. Sadly the AI decides that staying behind sometimes is more fun
But once the reinforcements stop you have basically won.
Its important to note this
1) Never ignore an alerted IV or III world next to you at the beginning, later on you can afford 1000-2000 ships on a planet like that and still sail through with minimal losses but at the start every MK3 ship is one too many.
2) Damage boosters, Use them. Every single multiplier doubles your effective fleet size (and MUCH more, if you fight against ships below your MK rank) for example having core ships munition boosted gives you a MASSIVE advantage over the ai.
3) Did i mention to never leave your home without Damage boosters? More damage means less losses for you. Its the most important part of AI War (currently, anyhow ,p)