Unit bias
The tendency to treat one serving, portion or item as the right amount to finish, whatever its size. We tend to complete whatever single package is in front of us.
In everyday life
People take far more candy from a free bowl when it comes with a large spoon than with a small one, because one spoonful feels like the right amount.
At the movies you finish an entire jumbo bag of candy, though a snack-size pack would have satisfied you just as much.
Why your mind does this
Finishing what we start is usually a good strategy, since a brain that quit at every bump would never complete anything. Valuing what we already have, what we built and what we worked for keeps us committed and protects our resources. Losing what we have can hurt more than an equal gain helps, so caution around losses often makes sense.
Its family, in 30 seconds: Finishing what we started
How to spot it
Notice when you stop because the container is empty and not because you've had enough.
How to counter it
Serve yourself smaller portions, or split large items before you start.