Well, Russia definitely has a lot of resources, especially manpower, or in Russia's case cannon fodder. And in that lies the whole mystery. Times have changed and the situation is not the same as it was in the USSR, and mass mobilization is not an option anymore because it would signify a war, not a special operation, and mass mobilization without a direct threat is hard to push on the people without retaliation (and we know what happened to Russia when people were unsatisfied with their governments waging a war). So using the biggest asset the Russians always had was the manpower, not the technology or the sheer firepower, and when that main resource gets taken away...well, we get this.
Not sure about the manpower part. Even with current population, Russia is one of the most sparsely populated nations on the planet. And to make things worse, the population is going down by more than 1 million every year, due to excess of deaths over births. During the Soviet times, the Russians could ignore the manpower shortage, because until 1988 or so, the natural population growth was much higher than that in Western Europe. But the collapse of the USSR changed that scenario. Now the fertility rate is only slightly higher than that in the EU, and higher death rate means that natural population decline is many times worse of what we have in countries such as Germany and Italy.