While I'm sure that large-scale mobilizations for the Russian Army are a problem (they always have been), the bigger problem is political. Putin wants to re-integrate the Ukraine back into Mother Russia. This is a "boil the frog" strategy designed to carve up Ukraine into a rump state for later digestion. Do it too fast and you risk Western intervention. Do it slowly & gradually and you're likely get away with it. The model is Hitler taking Czechoslovakia.
That is true as far as it goes, but you must keep in mind that Russia is one of the world’s largest nations with an economy only about the size of California’s. They’re big enough in geopolitical, WMD weaponry, and population to cause major problems, but too small in their economy and population to support their grandiose ambitions. They are also too corrupt and incompetent to run their own affairs without outside technological and financial support.