I am not sure where or how you guys are picking the stats but you are way over and in some case way under what would be true: here is an Idea for you to think about:
First of 8-11 range is "average". So everyone would have a range in this area for every stat except for some major, or even minor issues.
Strength 9-10 average, anything above 18 is "Olympic level strength"; My son is a power lifter i figure he has 18/75 or so strength) and he can bench 350, dead lift 750. a soldier who is on combat arms, specifically infantry would have a strength of 15-16, being able to carry a 35 pound combat load with no fatigue. It is possible to have a low strength because of lack of fitness, but anything below say 8 would mean some type of psychical issue, i.e. congenital deformity or acquired injury.
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Per the rules your son has a STR of 25 as he can lift a weight of 750 lbs and Hafthor Bjornsson str would be 36 or 37 as he can lift 1100.