Several of you have noted it already but it's to do with the diameter of the wheel bolts. Bigger diameter = more torque required. Hence the change from M12 to M14 bolts has lead BMW to a revise their torque specification from 120Nm to 140Nm.
140Nm is actually on the conservative side for what a grade 10.9, M14 bolt is designed to handle. As usual, all this stuff can be found with a quick web search.
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Realistically, the correct answer is "tight enough not to come undone but not too tight to strip the threads". But being German, they like specifics. Therefore 140Nm it is.