FJJP's theory about pillows is close..
But in fact the problem is the secret chemical the manufacturers use to impregnate Formal Dress. They know that it is worn less than any other clothing, so it won't be replaced very often. So they have another wheeze to boost sales.
Their trick is to dose the jacket and trousers with a compound which makes the garments shrink in the dark (in any wardrobe). So when we try them on next time they feel strangely tight.
I know that this is true.
A side effect is that when the jacket is warmed by the wearer, the chemicals used release vapours which, when breathed, give the wearer a headache next morning. As well as inducing certain memory loss.
Bloody tailors….