Pay for a grande, get a grande.

I always dump out any extra left in the blended immediately, so that if they ask that question, I can just say I measured it and this is the correct amount to give you, and I can point to the black lines on the cup to explain how the recipe works. My SM doesn't really care how we handle it as long as the customer leaves feeling ok.

At my store usually those people just want lots and lots of whip cream.

Had a customer the other day ask for a grande caramel frappuccino with 3 shots in a trenta cup with no whip.
I was thinking to myself which store are you going to where your drink fluffs up to a full 32oz lol

The proper way is exactly what you did. I would defend your actions here as a supervisor. Best response to when people say "they always do this" is, well they were doing it against policy and I am doing it properly to ensure your drink is as you ordered it.

My sm gave me the best cue card which was to reply with “Would you mind telling me who did this for you because that’s actually against company policy. We’ll need to correct this so that it doesn’t happen again.”
9 times out of 10 the customer suddenly either couldn’t come up with a name or description of the employee, or it “must have been another location.” I’m not saying this is foolproof but if they still persist with throwing a tantrum just repeat that what they’re asking is against policy and then go get your sm and let them handle it from there.

So you are wasting extra cups to ensure that you can throw the "extra" away? That is way more petty than my cheapest customer. Also, the way we "ensure they got what they paid for" is the drink recipe. Correct recipe may yield slightly extra volume on blended and shaken drinks. How does using extra disposables cups and treating demanding guests poorly helping our business or the atmosphere in our stores?

Quit wasting cups and use the cold bar measuring cup. Also lighten up and assume the best. They probably want the entirety of the shot they paid for. If policy was against giving excess in a larger cup they would make small drinks in large cups not a thing.

If you ever left extra room for an affogato shot, then you are a hypocrite. You should just pour it over the sides and get it all over your hand so you aren’t being stingy with the drink.

They policy is actually against giving excess in an additional cup because people would order a tall Frappuccino with extra milk and extra pumps and get a drink for free, since there is no way to up-charge it on the till. The excess is built into Frappuccinos to avoid shortchanging customers, not so we can throw it away and make customers upset for no reason at all.

Why are u guys so pressed about like 5 oz of frap? Sometimes there's extra, this person noticed so they asked for the larger cup. If anything it's wasting less product.

Here's my two cents - yes, the customer may be (mildly) wrong. But is the argument worth disenfranchising the customer? Often times the amount made can easily fill out the rest or the majority of a cup. In my opinion if Starbucks was striving to avoid this situation, they wouldn't offer the grande-in-a-venti cup option even on their own ordering app. Frankly, this isn't about any one side being "wrong" as much as it is not creating a customer service or optics issue by stubbornly and possibly malignantly clinging to what is exactly "right." As a customer I don't ask for extras, but admittedly more locations than not don't mind filling out the venti cup so it doesn't look comically disproportionate. I oddly find the places that will stringently "fight" you over proportions are generally lacking in other areas of customer service and are tone deaf when delegating their point of view. The customer who is paying $5/$6 a drink possibly several times a day isn't seeking an argument or even an impasse with the barista making the drink. And trust me, even displaying bemusement or confusion - not even outright complaining- can net you a condescending and nasty response at certain locations. At that point, the problem is becoming far more that you are alienating a repeat customer and far less that you are saving whatever money by micro-managing the proportion of drinks.