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On simple pharmacological principles for a given dose of alcohol, on average, a lean 80kg man will have a lower BAC than a lean 60kg man etc . If you add in (the reasonably wide) metabolic variation then it is quite possible for someone to have 7 drinks and still be in a low-range drink-driving zone. Such people will often not appear intoxicated (especially if they are habituated).
This doesn't mean that drinking enough to be habituated is a healthy lifestyle choice and might reflect a degree of alcohol dependency.
It also doesn't mean that if you are big you can jump behind the wheel of a car after 5 drinks-because of variation and habituation, it would be very difficult to predict a BAC based on size or apparent sobriety