A more acidic compound is one which releases h+ ion easily, i.e. has a weak bond between H and the anion. The bond between H and I in HI is weak due to the large size of I and low electronegativity. So HI releases H+ ion easily so is a strong acid. The bond between H and F in HF is strong due to small size and high electronegativity of F. So HF does not release H+ ion easily so is not a strong acid.