In terms everyone is thinking of, tanks and guns and planes, it’s a fraction of Russian size and power.
In terms of what matters most however, it may be another story.

Snake island, Ukraine
By now everyone is probably familiar with the story of Snake island. A small garrison of 13 border guards faced off against a Russian flotilla. They had no weapons with which to engage the ships and faced bombardment. They had no hope for success, no hope for anything. Russians called upon them to surrender.
Their response was “f**k off”*.
Russians then proceeded to obliterate the garrison with naval guns and missiles. Ukrainians preffered to die than to be captured by Russians, even when their deaths barely inconvenienced the invaders. Similar stories are dripping in from other fronts as well, Ukrainians are giving one hell of a fight with whatever they have.
Against such impetous better guns and tanks may well count for little. Russia learned nothing from Afghanistan and Chechnya it would seem.
*more accurately, the response was “go perform a fellatio”
The military forces of Ukraine possess fewer soldiers together with less military equipment than Russia possesses. The Russian military possesses more aircraft as well as ships and tanks than Ukraine. Ukraine proves its strength through different decisive efforts.
A perfect example of fighting spirit emerges from the stories of Snake Island. Ukrainian guards resolutely rejected Russian ships when they ordered surrender while speaking boldly to them. The high determination level characterizes Ukrainian military personnel.

Ukraine has some big pluses:
The military feeling runs high as soldiers want to defend their homeland.
Ukraine obtains military assistance as well as aid from Western countries through their support.
Their fighting strategy is characterized by intelligent plans together with innovative drone capabilities.
Ukrainian forces derive advantage from their familiarity with their territory along with receiving assistance from local population.
Despite having superior military resources, Ukraine defends itself from Russian invasion with great effectiveness. The Ukrainian military has both interrupted and significantly damaged many of Russia’s military operations.
Despite its initial appearance of dominance Russia faces challenges because Ukraine fights intelligently and receives support from countries across the world. The number of weapons possessed by an entity fails to guarantee military victory in combat.
As you can see in the graphic below, both armies have about an equal number of personnel.
In almost any other aspect, the Russians have a clear advantage: more tanks, more aircraft, and especially more artillery pieces.

(Source: Volodymyr Dacenko)
More important than sheer numbers, however, are other factors, many of them not quantifiable:
How well trained and motivated are the soldiers? How well-maintained are the weapon systems? Which side has the better intelligence organizations? How good (or bad) is the quality of the military leadership? Who applies the better tactics and who makes less mistakes?
In the end, the most important factors that will probably determine the outcome of the war are logistics and supply. If you are able to replace lost personnel and material, you have an enormous advantage over your enemy.