If I bought 20 inchers that only inflated up to 16 inches, I'd be livid.
That's actually a rather substantial size difference (More substantial that the difference between 16 inches and 12 inches).
Or I need to go back to buying my balloons from shops owned by looners. They may be a bit pricier, but they almost always have the freshest balloons in stock.
Amazon, on the other hand...