> i understand there is *some* dilution from stirring or shaking, but
> certainly it can't be enough to fill the glass, can it?

It can, but it shouldn't. There should be very little water in Martini.

So either bars use (too) small Martini glasses or shake/stirr too long
or with too little ice.

> if i were to make a martini for someone, and measured it carefully
> according to the recipe, would they feel cheated if the glass weren't
> full? do they come that way in proper bars?

I am a Martini lover, and I wouldn't feel cheated.

At least in Finland, when it comes to finer details of preparing and
serving liquors, bars are definetely not a good example to look at.

And as Martini is The Drink, there are dozens of different and equally
good ways to prepare them. But diluting with significant amounts of
water is not amoungst them...

Dale DeGroff's book The Craft of the Cocktail has a lot more info on
Martinis, if you are interested.