This is a question that comes up on nearly every fear of flying course that we run...
So why don't we?
We kind of do. Airlines use nominal weights for passengers. There is a weight calculation for adult males, adult females and children. This is calculated into EVERY commercial flight. It is well in excess of what is actually needed. That should reassure you.
Imagine the opposite:
Security: 'Sir, please step onto the scales...ah, 15 stones. Thank you.'
Pax: 'That can't be my weight - I 've been working out...'