It's a different war. Ukraine is able to stay in the fight due to drones and satellites.
You can't really mass your armies like you're Genghis Khan.
Sure, if you mass forces, you’ll get pounded.
Ukraine is taking large numbers of casualties, as are the Russians.
Trump mentioned in December 400,000 Ukrainians, and nearly 600,000 Russians as total casualties.
https://ukranews.com/en/news/1051760-trump-claims-ukraine-lost-400-000-soldiers-in-war-with-russia
Ukraine needs to achieve a 1:3.5 casualty ratio just to stay equal in their decline, but the best they are accomplishing is 1.5.
Ukraine will run out of warm blood-filled bodies before the Russians.
This is a war of attrition and if it does not end, it will be who can sustain the losses longer that will win.
This is the sort of war McNamara with his numbers and kill ratios could only dream of.