It all came down to the air bubbles.
Steve Jobs was known to prove a point in a memorable way, and that was certainly the case when Apple was designing the first iPod.
When engineers working on the very first iPod completed the prototype, they presented their work to Steve Jobs for his approval. Jobs played with the device, scrutinized it, weighed it in his hands, and promptly rejected it. It was too big.
The engineers explained that they had to reinvent inventing to create the iPod, and that it was simply impossible to make it any smaller. Jobs was quiet for a moment. Finally he stood, walked over to an aquarium, and dropped the iPod in the tank. After it touched bottom, bubbles floated to the top.
"Those are air bubbles," he snapped. "That means there's space in there. Make it smaller."
It's a lesson that seems to have stuck with Apple's engineers, too.
The original iPod was 19.8 mm thin. Years later, Apple's most recent iPod is only 6.1 mm thin, and all the space is used. Here's what the inside of the most recent iPod looks like.