@JMA: I think those in question are mostly those with some German background whose ancestors, called by the Tzars, emigrated a long time ago to settle down in Russia. Germans made arguably the biggest contribuition to the Russian economy, culture and military in the last centuries of all other ethnic groups. The 'right to return' was very attractive to escape the economic ruin and political prosecution in their homelands, 3 millions used that possiblity from 1988 onwards.

Certainly a huge demographic gain for Germany, especially since the intergration was relatively easy. At most three out of so many isn't much.