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Why is nitric acid stronger than nitrous acid?

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The structure of phosphorous acid, H3PO3. — oxyacid The structure of phosphorous acid, H₃PO₃.

The strength of an oxyacid is determined by the extent to which it dissociates in water to form hydronium ions. For a given nonmetal central atom, acid strength increases as the oxidation number of that central atom increases.1

In the case of nitrogen-based oxyacids, nitric acid (HNO $_{3}$ ) is a stronger acid than nitrous acid (HNO $_{2}$ ) because the nitrogen atom in nitric acid has a higher oxidation number of +5, compared to the +3 oxidation state of the nitrogen atom in nitrous acid.1