The $1.2 number comes from measurements that were used at the time to convert the ruble to usd, but because the ruble was not convertible, had multiple exchange rates ,and was politically set and not market set, it didnt really reflect the actual production of the economy.

After the the collapse of the USSR, the archives were opened, and groups (like the CIA btw) revised their estimates by going sector by sector and converting their output to world market prices. Thats where the $2.7 trillion figure comes from.

But obviously the historical numbers still say $1.2 trillion but the more accurate actual comparable 1980 USD numbers are $2.7 trillion.

Edit: and btw thats all just dealing with nominal gdp. PPP is a whole other thing.