Big numbers both ways.
Anyway, with Germany refused for a total European Victory, and under an embargo and bombing, the USSR was slightly too big a climb.
Also bear in mind, that had Britain not been an enemy of the Axis, Japan's next go would possibly have been the already weakened USSR. After all, they'd been enemies since 1904 or even before.
Japan did indeed consider this option, but decided on the big game and went for the USA/UK.
That descision saved Moscow, and with Moscow fallen, there might not have been any Stalingrad....