Helmet strap loosens as I ride then the day after becomes too tight
Two One hypotheses:


The straps are getting wet with sweat and loosening, then constricting again when they dry. Nylon does in fact absorb water and stretch when it gets wet, so this is the more likely cause. 


2. Exertion causes vasodilation. If you've had the sensation of your necktie getting tighter under stress, this is the cause. Your body is pumping more blood to your head and the veins are expanding.