Another issue is that if you're attacking an enemy planet, it's largely because you want what that planet has reasonably intact. Which is another factor cutting against the "nuke 'em till they glow" line of thought, because all the infrastructure and resources you destroy by glassing a planet are infrastructure and resources you don't get access to.
To add to this: ground to orbit weapons don't have a yield limit. While orbit to ground weapons
would have a yeild limit simply because you don't want to glass the planet(and if you
do want to glass it just tow a fucking space rock into it, much cheaper and would take a lot less time while being harder to stop).
A lot of ground combat in sci-fi is kinda unrealistic because you'd need hundreds of millions minimum to garrison a force and routine reinforcements due to insurgency and rebellion unless you know you do genocide.
40k and Star Wars is like here's a thousand or a million guys go get em kiddo and that's it.
A couple thousand people
could be enough to hold a planet when combined with a carefully curated police force.
Once you control the world, you don't have a primarily resistant government fighting you: are are that world's government. You control the police forces, which is one of the obstacles of taking a place to begin with(local and organized resistance) which means your dedicated soldier requirements go way, way the fuck down. Instead you replace the old police force with a new one made out of people that are sympathetic to your cause or empire from the existing civilian populace.
The 1k-10k actual soldiers you have on world would be there as a fast reaction force to any full scale rebellion, the smaller insurgency level stuff can be handled by your new police forces.
This has been how you successfully conquer and occupy a city or country for the last 8 thousand years; I don't see why it would change because it's now a planet instead.
Then it makes you wonder, why the hell bother fighting for these sparely populated areas lmao 😂
I remember one episode where the Separatists invaded a planet that had stone age technology (literal stone huts)...for reasons (didn't even mine it) and the Republic fought over like three major cities.
That was to capture slaves and sell them for the big bucks.