HI is stronger acid than HCl is because of heat of ionisation which is large for HI(63.8KJ/MOL) compared to HCl(59.8kj/mol)

HEAT OF IONIZATION : the energy released in ionization of 1 MOLE HX (X=HALOGEN)

HClO3 is stronger acid than HIO3 because as the electronegativity of the central atom X decreases as we move from CL to I , the tendency of the central atom to attract the electron pair of X-O bond towards itself decreases and hence the possibility of rupture of O-H bond decreases.