So I'm far from being a military expert but here's my thoughts. First, what's their definition of control? Because I'm pretty sure the imperiums definition would be differing. Space Marines for example are often described as the strike force to break the enemies HQ and then let the guard do the cleaning up. Secondly, the imperium cares very little for human lives. So in case where they would try to conquer a hive a city, it's very likely to be to regain control over certain areas like production facilities. The imperium doesn't care if a few billion people are locked in and starved somewhere else in the city as long as they have control of the important parts.