I think most did not under estimate the Ukranian forces.  Like you said, Ukraine has 1/2 million troops, lots of weapons and has Western support.

Kiev has a population close to 3 million. Spralling city.  Do you think  the small amount of Russian troops inititally amassed to "storm" Kiev will work?  Refer to the map below.  Obviously, the aim is to create a buffer zone at the Russian border and if possible, annex Odessa to landlock Ukraine and full access to the Black sea.  The only warm water sea for Russia.  Kharkiv is the hardest one for the Russians.  Lots of Russians died trying to take Kharkiv.

Since Eastern Ukraine are mostly Russian speaking Ukranians,  Russian troops have to be surgical in their attacks and try not destroy existing infrastructure.  It doesn;t get any easier when the Azov troops are using civilians as human shields.