One pill or two pills?

One pill, two pill, three pills, or the bottle?

I had a headache and the aspirin bottle gave instructions of 1 to 2 pills for adults. I looked at a package of pills for my dog and it said based on weight give this dosage. Now both people above are adults but one is nearly 6'8" and the other is only 5'6". So is one person going to be under medicated and the other over medicated? This is a prescribed dosage set by the FDA, so can you trust what the FDA says? What else did they get wrong? If the dosage is so general, do we really need the pills in the first place? Are our pets getting better medication than we are? Is everything an illusion?