more acidic has got nothing to do with stability,,it justs reflects the ability of acid to donate proton

 

isnt ClO4^- better O.A than ClO-...coz its oxidation state of Cl in ClO4- is +7 which can down by self reduction

 

 

strength of acid as i said is ability to donate proton..since in HI the H-I bond is larger...its bond enthalpy is less, hence easier to ionize...releasing H+ easier..making it stronger acid