Tonight we ate at a cool little diner. I ordered a nice glass of syrah from the bartender. Later, our food server asked if I wanted another glass. I said yes. She proceeded to pour literally half the amount I got the first time around. I've had this happen to me numerous times in the past, and there never seems to be a solution. What does one say without sounding trivial or annoying?
The depressing short pour – I share your frustration. By drawing attention to the matter, you obviously risk being objectified as a lush, cheapskate or just plain jerky. But you have every right to expect a second glass equal in volume to the first.
The presence of two pourers complicates your situation. A complaint will have the effect of painting the hard-working waitress as chintzy next to Free-Pour Willy behind the bar. It could backfire, too, assuming Shot-Glass Sally is in no mood for a challenge. She could argue that the bartender – not she – was in error with regard to volume, that you were already ahead of the game with that liberal first pour.
Perversely, there is no guarantee of consistency at most fine-dining establishments. Virtually all but the cheapest places use glassware much larger than the serving size. The point here is to provide headroom for swirling and sniffing, which is a good thing. Five ounces should be the standard pour, but how do you know what you're getting?
The upsetting issue here is that most bartenders and waiters do not use a proper measuring vessel. They simply aim for an imaginary line somewhere up the side of the glass and stop when they think they've reached it. You'll notice they usually do this with a calculated air of conviction, which is part of the game ("I'm a pro"). But human servers do not have the precision of government-inspected gas pumps. (Imagine if they sold gasoline using the rough-guess method!) As we all know – and as you have noted in your question – the imaginary line has a funny way of moving up and down based on the person and the moment.
Curiously, this doesn't happen much with beer. You know you're getting a consistent volume of brew each time you order – either a full glass or a standard brewery bottle.
I would speak up – but respectfully. Try something like: "I don't mean this as a complaint, and I'm having a great time. But I was hoping the second glass would be just as substantial as the first. Am I being rude if I ask for a splash more? If so, don't worry about it, I'll survive."
Remember, servers tend to earn most of their income from tips. They want you to feel satisfied. When they underpour, they usually do so inadvertently. (It's the owner who stands to gain from skinflint service, not the salaried server.) They'll probably conclude that an extra ounce or two of moderately priced wine is a small price to pay for a patron's satisfaction. If the server balks, at least you've got your frustration out of your system. And you've got the last laugh: It's called undertipping.