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H Cl O4 -  PerChloric acid
H Cl O3  - Chloric acid
H Cl O2  - chlorous acid
H Cl O  - chlorous acid

   Acid dissociates  in to  H+  and Cl On)⁻¹  ions.
The anion  (Cl On)⁻¹  is biggest in size for PerChloric acid.  Thus the attraction and binding of H+ ions to it is the least.  So, it can donate the proton more readily than other acids.
You can also find acidity strength based on oxidation states.

Based on size,  acidity strength, in increasing order is :
         H Cl O ,  H Cl O2, H ClO3,  H ClO4

1 5 1