Expansion is limited as most Hives are relics of the Dark Age of Technology, so their construction methods on a macro level are forgotten and would take hundreds, if not thousands of years to build...
Since when does the imperium have a shortage of time on it's hands? But all this is beside the point. These are worlds defined as having a substantial portion of their surface area covered, kilometres deep, in slum-sprawl. If that much civil infrastructure were not substantially inhabited and actively maintained, they would have been salvaged for materials or collapsed into ruins long ago, and that implies a population in the trillions.

I'm sorry, but there is no working around this. If hive and forge worlds exist at all, they must inevitably dominate imperial demographics. (And they must inevitably be largely self-sufficient.)

No. of confirmed Hive Worlds x Minimum Necessary Hive Population:
35,000 x 3 Trillion = 105 Quadrillion human beings.

No. of confirmed Imperial Worlds X Typical Pre-Industrial Population:
1 Million x 1 Billion = 1 Quadrillion human beings.

Space Marines are best at striking their target hard and fast. When they do that you only need a few (a few dozen Companies take on basically all the Dark Eldar at once - in Commoragh - and most of them got out alive)...
Which, like I said, is numerically nonsensical. The sources may state otherwise, but this only implies that the sources are spouting nonsense.
And I've provided many, many examples of it not being that bad in what I've show are statistically more likely scenarios...
They're not statistically likely. They are, on the basis of eminently conservative mathematic, demographic and economic assumptions, insignificant compared to hive world scenarios.