(Accidentally deleted trying to make an edit from my phone... Here is prior comment)
To be fair it would have to measured as a proportion of the country's land mass. 1 Km² is a lot more of Ukraine than 1 Km² is of Russia. Russia has tons of land, most of which no one wants anything to do with. Ukraine has much less and it was mostly good land before Russia starting fucking everything up.
EDIT: was curious, so I looked it up.... Just counting landmass on a proportionate basis (not economic value of the land, it's arability or any other factors) each Km² in Ukraine is equivalent to 27.13 Km² of Russia. (Russia: 16,376,870 Km² / Ukraine: 603,628 Km²)
Finding the extent of environmental damage in Ukraine was kinda difficult, but there were multiple discussions on how munitions cause lasting damage to arable soils, and discussion of how Ukraine had a high concentration of Chernozem, a type of soil know for being very fertile and valuable agriculturally.
This link (Ukraine Has Largest Minefield In The World, Prime Minister Says ) estimates damages so far:
According to Ukrainian Prime Minister Denys Shmyhal, the Russian war in Ukraine has created a minefield of 250,000 square kilometers in size in his country. "It's currently the largest minefield in the world," Shmyhal told South Korea's Yonhap news agency in an interview published on January 8. The mined area, according to Shmyhal, is equivalent to more than 40 percent of Ukraine's total land area.
Just going based on the extent of minefields, for parity, 6,782,683 Km² of Russian land would have to be similarly destroyed and rendered uninhabitable/unusable. Since Russia makes up about 11% of the landmass on the planet, the extent of this devastation would be equivalent to about 4.55% of all above sea-level terrain on Earth.
This would be roughly equivalent to the combined landmass of the largest 20 US states: Alaska, Texas, California, Montana, New Mexico, Arizona, Nevada, Colorado, Oregon, Wyoming, Michigan, Minnesota, Utah, Idaho, Kansas, Nebraska, South Dakota, Washington, North Dakota, and Oklahoma.
Ecologically, it's hard to imagine the consequences of devastation on that kind of scale. Maybe reciprocity isn't the answer in this case. A better solution might be for the international community to confiscate that land from Russia as a preserve or something, until such time as Ukraine has been repaired and made whole from the damage Russia has caused.