[jschwartzi]

Intercostal muscle tension probably has a bigger effect than thermal expansion of the tape. The intercostals are the muscles that control how close together your ribs are. They're auxilliary muscles for breathing. Diaphragm and core muscle tension will also have an effect. Your pectoralis muscles and trapezius and deltoids will all also have an effect. These are all muscles that control the size and shape of your ribcage. The human body is not a static shell, but flexes and changes shape depending on the tension of different muscles.

When we touch cold things to our skin, we naturally tense up. If your intercostals, diaphragm, and core are tense when you're being fitted for something that is snug against your chest, then when you get it out of the box it will be too tight.