Every scoop that goes over the rim is ice cream you gave away. It never appears on a report, because nothing was recorded and nobody complained. It simply left the building.
Over a season, at the volume a busy counter runs, that adds up to a number worth caring about. And the thing controlling it is not your staff. It is the cup you handed them.
The cup is the portion
Nobody scoops to a weight. They scoop to a container, and they fill it until it looks right. That means the cup on your shelf is setting your portion size whether you intended it to or not.
The 6 oz paper ice cream cup is a standard single-scoop portion. Hand someone a 6 oz and they produce a single scoop. Hand the same person an 8 oz for the same order and the scoop grows to suit the space, without anyone deciding to be generous.
Where the single scoop belongs
This is not an argument for meanness. It is an argument for a menu where a single scoop and a double scoop are actually different things.
If both come out of the same cup, the customer paying for two scoops is looking at a cup that resembles the one-scoop order they had last week, and they will draw their own conclusion about your pricing. Separate cups make the difference visible before anyone tastes anything. The 6 oz for one, the 8 oz for two, and the gap between them does the explaining.
The training benefit nobody mentions
New staff cannot judge a portion. They have no reference for what a scoop should look like, and telling them is far less effective than showing them.
A dedicated cup per portion size removes the judgment call entirely. Fill it to the rim and you are correct. That is a rule someone can follow on their second shift without hesitating in front of a queue, and it produces consistency you would otherwise spend months building.
Color, briefly
The 6 oz runs in solid colors, polka dot prints and striped prints, and every colorway is the same cup body at the same case price.
If you are running it as a portion-control tool, there is a strong argument for making it visually distinct from your double-scoop cup rather than matching it. A different color for each size means staff grab correctly by sight during a rush, and a customer can see at a glance which order is theirs when two go out together. We covered the two color strategies in choosing an 8 oz cup, and portion control is the case where the second one wins.
Working out the split
Most counters find the single-scoop size moves slower than the double. Order accordingly rather than matching case for case, and adjust after a season when you have real numbers instead of assumptions.
The full ladder, from the tasting size up to the party size, sits in the ice cream cup range. If you currently run one cup for everything, adding the size below it is usually the cheaper improvement, because it costs you nothing per serve and stops the giveaway at the top.