Possible scale of war in Ukraine [closed]

The size of the Russian military build-up near Ukrainian border and the possible scale of war have both been repeatedly claimed to be most significant since World War II. For example (emphasis is mine):

Reuters (February 24):

How Biden handles the crisis, which Western officials fear could spiral into the bloodiest European conflict since World War Two, is expected to have profound implications for his political fortunes and U.S. relations with the world

Politico (February 24):

His pronouncement at 5:45 a.m. Moscow time — during a simultaneous U.N. Security Council meeting in New York, at which Western nations pleaded for Putin to exercise restraint and de-escalate — could spark the largest land war in Europe since World War II, one that could result in the deaths of thousands of Ukrainian and Russian troops and civilians, and spark a refugee crisis.

CNBC quoting Joe Biden (January 25)

WASHINGTON -- President Joe Biden cast a potential Russian invasion of Ukraine in stark historical terms Tuesday, saying, “it would be the largest invasion since World War II.”

BBC quoting Boris Johnson (February 21)

Evidence suggests Russia is planning "the biggest war in Europe since 1945", Prime Minister Boris Johnson has said.

What is the basis for such assessment? For example, in terms of the scale of fighting, the level of NATO involvement and the loss of life, this war (even if counting from 2014) is still well behind the Yugoslav wars in the last decade of the XXth century.

Remarks

• The question is about fact-checking (regarding the preceding Russian military build-up) and the probabilistic/intelligence/military assessment of the outcome of the ongoing military conflict.

Background
Yugoslav wars
I have already mentioned the Yugoslav wars, which resulted in 130,000-140,000 deaths and millions of displaced persons. In Croatian war alone the numbers of combatants for the two sides peaked at 200,000 and 145,000 - which is comparable (or greater) than the alleged Russian pre-war build'up. The war in eastern Ukraine (since 2014) has resulted in about 14,000 casualties so far.

Czechoslovakia in 1968
Another interesting example is the Warsow Pact invasion of Czechoslovakia in 1968: while the number of casualties was relatively small (about two hundred deaths), the initial troops build-up of 250,000 exceeded the reported Russian strength in the current crisis. At its peak the invading force was about half a million strong.

Thus, the claims regarding the Russian pre-war build-up are questionable on the factual basis, while the projections regarding the expected number of casualties are likely exaggerated.

Similar questions (requests for reliable information
Is "100,000 soldiers near the Ukrainian border" a rare occurrence?
What's the basis for preparations to imminent Russian invasion into Ukraine?
How close is supposed "Russian build-up" to Ukrainian border?