Leaving capacity of halogens
When HF loses proton the conjugate base formed is F- Fluoride anion,Since fluorine is small and compact atom having lower affinity to keep electrons because of lack of d orbitals thus fluoride ion is unstable conjugate base compared to iodide ion which is stable.Happy conjugate base(stable), more reactive acid. Hence HI is more acidic than HF.

Even though fluorine is highly electronegative(tendency to drag shared electrons in a covalant bond),it has lower affinity to keep this electrons(lower electron affinity) due to its smaller size .