My dad was 6'2/245 lbs - he was huge but I would not say he was a "fat fuck."
He was pretty evil though, which is far worse than a person's appearance in my book of life. : )
People of your height/weight (or BMI) are typically included in drug studies and are therefore accounted for in product labelling (and this would apply to propranolol as well).
Genetics are an odd beast. IME in the pharmaceutical industry, there have been many instances of meds not working when they clearly should have given their mechanism of action or at the opposite extreme (albeit more rare), meds that kill no matter how benign the med seems to be - like an otherwise healthy 26-year-old getting an antibiotic for a sinus infection, which, after taking a couple of doses caused multi-organ system failure and death - and it was not an allergic reaction, which are well-known with antibiotics. He should not have died - and at the time, there was no way to do genetic testing to see what happened. (It was so very tragic to see what happened to him - but it does happen).
Maybe it just boils down to different genetics or a strong constitution where you do not feel the effects as much as others do? And by constitution I mean a higher threshold to withstand potential side effects (similar to those who have a high threshold for pain)?
<3