Due to the lack of actual data in Sins, I've begun theorycrafting numbers.
A fully upgraded terran planet has 6000 HP and a population probably superior to that of Earth today, observed through the illumination of civilization on the planet's dark side, as well as the massive ( obviously exaggerated but still count as massive ) cities that can be seen from orbit.
For the sake of having something to compare it to, we'll assume it's got the population of Earth.
Now, the 6000 damage represents the amount of bombing damage it would take to wipe out all human/advent/vasari life on the planet ( so not a complete sweep of all living things ).
A Tsar Bomba, weighing in at 50 megatons, has a fallout area of 1000 square kilometers. Human civilization spreads over 18 million square kilometers.
It would then take 18 000 Tsar Bombas, or 900 000 megatons, to wipe out all human life on Earth.
What's this mean? Those 900 000 megatons are represented by 6000 health points on a planet. Which means each point of health ( and therefor, damage ) represents 150 megatons of destructive force.
So, an Advent Seeker Vessels, which has an indicated 2 damage-per-second ( it's damage per second right? I'm not sure how the numbers on the infocards work in relation to weapons firing / quantity of weapons / cooldown time ) is actually dishing out 300 megatons of firepower every second, and that's just a scout ship.
This might sound excessive, but Sins is in the far future. The Vasari have been around far longer than humanity and have developped their weapons long ago. The Advent were really damn pissed and spent 1000 years focusing on finding better ways to eradicate the TEC. And the TEC, well they just had to match what the other two races were putting out if they wanted to survive, so they built damn big guns. Really damn big guns, as demonstrated by the 20km long rail cannon that we call the Ragnarov.
This speaks for the shields and armor in the game as well, since they withstand all that firepower several times over.
Now you might argue that there is no link between planet health and ship health, but planets have their own set of weapons, siege weapons, that can only be used against them, and no weapon used in ship-to-ship combat can be fired against a planet.
Basically, if they didn't want this kind of comparison, they could simply have given the planet 100 000 health and given the siege weapons higher damage values and the game would have been exactly the same.