In case of hydrogen iodide the bond length is larger as compared to hydrogen fluoride. As the size of fluorine is smaller than iodine, the bond between hydrogen and fluorine is stronger and have lower bond length. This makes it harder to break the bond and release H+ ions.
But in case of Hydrogen iodide the overlapping of orbitals is not satisfactory as the size of iodine atom is much larger as compared to the size of hydrogen. as the size of iodine is larger the bond length also increases and it makes easy to release H+ ions and show high acidity.
The Unconditional Guru 