The strength of acids r usually correct when compared by the stability of the conjucate ion formed after acidification....

P is in +3 state in H₃PO₂ the structure contains onlyone OH group rest all atos r attached to P directly(O wid double bond)...

H₂PO₂^- is more stable than H₂PO₄^- coz of less repulsions between the bulky grps as in H₂PO₄^-..... the same is the case wid HSO₃^-.(which has an angle of 120^. between the groups...