that 30 certainly is below normal in general, especially for somebody who's not little. Ideal is said to be 115/75.
Do you get dizzy spells or fainting? Headaches? If you have no problems like that, then maybe you're just made to be at a lower blood pressure. One concern might be if the organs aren't getting enough blood/oxygen.
Can you think of anything that you do or consume that's not usual? Have you had a recent infection? Are you dehydrated?
As long as you are asymptomatic, your much-lower-than-average diastolic readings are nothing to be concerned about. Height and weight are not factors in determining desirable BP targets.
A resting heart rate of 55 is possibly indicative of bradycardia, although healthy young adults and athletes often have heart rates of less than 60. "Asymptomatic sinus bradycardia requires no treatment", according to the American Heart Association web site.