Big consideration: the USSR's industrial economy was either larger than that of Nazi Germany or at least similar in size. China's industrial economy was smaller than Japan's, especially after the loss of Manchuria (which was China's most industrialized region by an almost comical margin) in 1932.*
And then there's the matter of where that industry was located. While the USSR did lose some important industrial centres in 1941, many of it's most important centres were already deep within the interior, and the German advance wasn't fast enough to prevent the relocation of many factories to areas firmly out of their reach. China's industrial centres were, with a few notable exceptions, clustered in the north or along the coast, meaning they were quickly overran once the war got underway, there were some efforts to relocate vital industries (most notably the successful relocation of the Hanyang Arsenal in 1938) but in general the balance between saved and lost was very bleak.
*this also meant that Manchuria's industrial potential was put towards the Japanese war effort, further widening the gap
Paul Kennedy, The Rise and Fall of the Great Powers.
In 1937 the top 5 were:
The US was at 41%
Germany at 14.4%
USSR at 14.0%
UK at 10.2%
France at 4.2%
Aren't those %s for "war making potential" or something like that? So not just industrial output?