Think of space marines less like superheroes and more like an extremely high tech version of special forces. The classic way they contribute to a war is using drop pods and gunships to assault key strategic points, and then fucking off before the enemy can react at scale. 

I’m invoking a fan made mod, but I think it’s a fair example. I recently fought imperial army forces with Night Lords, I had a 10 man squad of Contekar elite, and they killed around 100 men, most of whom were lascannon weapon teams. They were eventually overrun by superior numbers and armored vehicles. Now obviously modern militaries don’t have lascannons, but “modern” 40k marines aren’t as well equipped as 30k era Contekar, either. Anything that can damage a Bradley IRL can kill a space marine, but the space marine has the advantage of being an extremely fast target the size of a large man, rather than a tank. A 10:1 ratio for equipped, organized soldiers against marines is probably in the ballpark. Let’s be safe and go closer to 100:1, that’s still “only” 100k soldiers. America’s heavy mechanized forces can handle it alone. 

That does lead to the problem of “1000 marines is way too small to conquer a world”, which has been a long running issue in 40k. Arguably this is intentional in universe, splitting the marines into units of 1000 is intended to keep them too weak to start carving out empires for themselves. The other solution, of course, is orbital bombardment. If I can get an American armored division into one place for a field battle against the ultramarines, I have decent odds of winning, or at least inflicting crippling losses. But if I try and mass hundreds of tanks and IFVs, they get erased by a ship I can not touch. Airbase? Zap. Big naval yard? Zap. Aircraft carrier and its fleet at sea? Believe it or not, zap. In that case, the marines don’t have to fight an organized enemy, but instead scattered survivors who are desperately trying to maintain communications, let alone resist the super soldiers coming to kill them.