Some back-of-the-envelope calculations suggest that first number is in the right ballpark. From [0], the lift generated by helium vs Earth air at STP is approx. 1.03 g/L. This suggests a lift of approx. 590,000 short tons on Earth:
Then, if we assume that the gravity on Venus is 90% of Earth standard, you get about 656,000 short tons of lift. Note that this doesn't take into account the different air density on Venus.
http://www.wolframalpha.com/input/?i=%28%28%28%284%2F3%29+*+...
Then, if we assume that the gravity on Venus is 90% of Earth standard, you get about 656,000 short tons of lift. Note that this doesn't take into account the different air density on Venus.
No idea about the "doubling" bit, though :-P
[0]: http://www.chem.hawaii.edu/uham/lift.html