Not to mention when Russia annexed Crimea, in which the expectation was that Crimea was welcoming (or "welcoming", point is they aren't welcoming the Russians w

Not to mention when Russia annexed Crimea, in which the expectation was that Crimea was welcoming (or "welcoming", point is they aren't welcoming the Russians with arms of their own), they sent 40k troops, to somewhere with an area of 27000km^2. If you do a dumb-as-a-post scale up of that to Ukraine, purely with troops / area, you'd expect a lot more than 500,000 troops; and unlike Crimea, such an invasion will be met with resistance.