My dad was the 7th child meaning that his dad did it 6 times bfore, wat r the effects?

ok my granddad had 6 children before my dad, now my dad was 7th, btw u should kno that is the old days parents had a lot more children. anyway now im a 19 year old boy and im rlly weak compared to other boys and i hav a small , is that because of the things i mentioned?