Kiev is roughly the population of LA. LA has 10,000 police officers. Think about how many more would be required to take and hold LA. 100,000 isn't enough if they want to occupy the cities. That's why I don't think it is likely.
A quick look, in 1941 Army Group South was tasked with taking Ukraine. The Germans used about 30 corps (plus many smaller units). A German Corps had three divisions, German divisions had at least 12,000 men (early in the war before casualties reduced that greatly). So that means Germany used at least 720,000 men to take Ukraine in 1941.
Now technology has reduced those requirements greatly. But for taking and holding cities, I doubt it has reduced it enough that 100,000 is sufficient. Fighting in modern skyscrapers cannot be easy. I'll let Ranger address that issue as I am positive he knows far more about urban combat requirements.
My current belief, the Russians believed that the Ukrainian government was not popular. They believed a quick incursion would activate mass numbers of Ukrainians to flock to their cause and topple the current government. This sort of thinking has happened often, in their invasion of Kentucky the Confederacy was worried about what they would do with all the volunteers they were going to receive.
It may be they haven't had a real plan to take Ukraine, it may be they had a plan to trigger a revolt and that plan has failed. And I'll back that up with a commentator this morning on CNN suggesting the Russians were doing poorly because Ukraine still has an air force, still has armored formations, still has command and control. In US doctrine both would have been disrupted before entering. But in my theory, the Russians wanted command and control to survive to order troops to surrender. They wanted not to destroy the Ukrainian military as doing so would harden the attitudes away from Russian support. Russia thought a quick trip in and Ukraine would fall.
One thing we don't know, how many of those troops are from Belarus. Russia may have used more Belarus troops and those may not be as motivated/good.