Official cocktail recipes vs typical glass sizes
Despite what many bars, convenience stores, and fast food restaurants would have you believe, drink glasses are not intended to be "filled to the brim" (or even close). This is particularly true of cocktails and other spirits. The 'head space' allows the drinker to swirl (or, if they have had a few, 'slosh') the drink around to remix the cocktail ingredients as they seek the bottom of the glass. Any water that has melted into the drink will be diluted into the mix, and the drink may be more directly exposed to any remaining ice in order to chill the liquid just as it is consumed. Thus, the 'standard glass' has been designed to exceed the standard drink. Of course, always wanting 'as much as we can get' people tend to expect their neat scotch to be filled to the brim...just like the self-serve uber-drink from the local gas station. 

In the absence of an official rule book for pouring drinks into glasses I can offer some references you might look to, including the fact that a Google Image search of "Full Cocktail Glass" almost universally shows images of drinks with significant air on top. Commentaries from self-appointed experts at the bar as well as amateurs chiming in with their thoughts tend to corroborate this view. For my part, if more than two members of the family say the word "reunion", Jack Daniels stock jumps five points...so I can at least pretend expertise... 


Even a beer glass (a pilsner) is designed to make room for a head on the 'proper' serving.