That is not necessarily surprising. GDP is the sum of the value added by four institutional sectors: government, corporations (financial and nonfinancial), household and nonprofit institutions serving households (npish). However, it does not reflect value created through non-market transactions (the so -called informal or grey economy. Given the enormous size of the informal economy in Eastern Europe, especially Russia, one can easily "increase" the GDP by that much by accounting that assigns a value to the informal economy or its part.
The pre-1989 estimates of the informal sector in Poland I saw gave the figures in the vicinity of 12% of the GDP. I would assume that in the ex-USSR that figure was much higher (perhaps even double), and increased considerably after 1990. I would consider it believable if someone estimated that figure to be 30-40% of the GDP. With that in mind, accounting for half of the informal economy can boost the GDP by 15-18% without any increase of the actual value added.
Any thoughts? Barkley? Brad? Michael P.?
Wojtek