Here's a data point for those interested in ground combat...
In one single-player game with a +10% ground combat bonus, I've been able to capture planets of 8 *billion* people with all ground defenses (e.g. 13-15 million tons of weapons platforms) without orbital bombardment of any kind.
Traits relevant include the aforementioned GC bonus, plus a +10% defensiveness bonus -- which will help a LOT.
Transport design is a heavily shielded (maybe 2400 pts or so -- most of my warships lean towards firepower, instead) battleship, with full training (+20%/+20% fleet/ship), ECM III (+60%), Stealth Armor and Scattering Armor (+25% from those two, IIRC). That's a total of... +135% defense if I do my math right; there's no size penalty for being a BB. Subtract 65% figuring the weapon platforms may have a Combat Sensors III; that's still +70% to defense net, *plus* range penalty (since these transports have QE IIIs plus Solar Sail, they can stop at range 7 before closing next round). Net effect is that enemy hit rate on transports appears to be 1% (warning: not against Deeply Religious races...). Units can't gain experience, which cripples them (again, non-Religious) when attacking a prepared fleet, so in practice these transports have yet to sustain more than perhaps 2-3 hits to shields even against huge amounts of ground fire.
Note: Refitting a Legendary warship in a Legendary fleet to a combat transport can bring total defense bonus to +195%; +130% less CS III. Subtract 10% for a race with normal defensiveness; at that point, it's not particularly necessary...
Each transport (perhaps up to 4 in any given task force, each of which contains several warships, plus sweepers, shipyards, scanner, and warpships; they're detachments from the Grand Armada) can transport 162 Large Troops, each of which has ECM, Sensors, 2xShields, and 3xGround Cannon.
A single wave of 162 troops, in my experience, will trivially conqueror (1 round, no return fire) many lightly populated planets. For an completely full, 8B world (400 militia; in test case, pretty much full of weapons platforms, 'tho I don't know if that affects GC), the first wave sustains heavy casualties, but survives the 10 turns; a total of three drops overwhelms the militia, with perhaps ~240 losses among the attackers. Two waves might suffice, but probably with greater losses, and also it may take more than one game turn. Multiple transport landings should take place simultaneously if at all, to minimize time spent under fire. Moons should be taken simultaneously as well, if they have WPs that could fire at your new acquisition...
Incidentally, the planetary WPs can be fired the same combat turn you capture 'em. Assimilation is quick.
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-- The thing that goes bump in the night