The number on the BMWNA site is a typo. Unladen weight for the European spec 135i is 1560 kg. Note: that includes 75 kg for driver and luggage (as do all BMW curb weights to date!).
That's 3440 lbs. I'd expect the US model to be the same or more due to a greater level of standard equipment (electric seats and moonroof are not standard on the Euro model) - though I don't know if BMWNA is still using the EU convention (curb weight plus 75 kg for driver/luggage) for listing their unladen weights.
Therefore, the difference in mass between the 135i and 335i isn't particularly large. I think it's vastly overhyped, myself, and it's difficult to see what real incentives this car offers over the 3er. The 123d is a more interesting product.
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