Basic nature of an element's oxides depends on the electropositivity of the element.
Electropositivity of an element is the measure of the tendency to lose the electron pair when it has participated in formation of a polar bond (polar covalent or ionic both)

Electropositivity increases with increase in size of the atom as the effective nuclear charge decreases with increase in atomic radius.

In a group from top to bottom, the electropositivity of elements increases from top to bottom as the size of atoms increases down the group.

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