One of the big differences is that Ukrainian NCO's and Officers have been getting trained by NATO countries since 2014, and that has accelerated. So, while Russia is burning through it's comparatively small number of trained troops, and utterly lacking in NCO's, Ukraine isn't losing ground organizationally. They've surely suffered some terrible losses as well, but they're a more professional force and so are better able to integrate new recruits.
That qualitative/professional component is huge.
Russia has only been able to use volunteers and paid mercenaries under the terms of SMO.
In a few weeks that could change.