Because Ukrainian soldiers are not unlimited, and Russia's population is about 3 times larger.

Plus, at some point Russia will start using nukes and then we will be forced to back down. We are not going to blow up the world to save Ukraine.

Ukraine is fighting an existential war. It has far more to lose and therefore can and will commit far more of its population and resources to the fight.

Russia has a greater population, but has been relying largely on conscripts among the poor, the incarcerated, and third world nations likes North Korea. Thats a resource that is far smaller in number than their total population. As soon as it’s tapped out, they either have to give up are start conscripting people in the middle and upper classes, and that will turn the Russian population on the war and their leaders faster than anything.

Right because countries with smaller populations can't beat larger ones when defending their homeland. Oh wait that's exactly what happened in Afghanistan vs USSR and USA, Vietnam etc.