I don't think Ukraine has lost as many troops as Russia has. Perhaps you saw that Ukraine has high casualties and thought that meant "killed"? Because casualties is a broader term that includes killed, but also includes wounded, missing, or otherwise out of action. I could believe Ukrainian casualties are 125k maybe. Many do get wounded. But unlike their Russian counterparts, they often get proper medical treatment and many even get to go back into the fight. Russians get left to die. Likewise, Russians killed is definitely over 100k by now. The rest that counts as casualties is likely an even bigger number.