stability of a negative charge in halogens and that of orbitals when we talk about stability of halogen anions we say,
$\ce{I-} \gt \ce{Br-} \gt \ce{Cl-} \gt \ce{F-}$, and the reason is that &quot;the negative charge (the electrons) have more space to move about in the larger halogen&quot;. But when we talk about stability of a negative charge on an orbital, why do we say that the negative charge on the $sp$ hybridized orbital (which is the smaller one) is more stable than $sp^2$ and then $sp^3$ as the least stable where as the  $sp^3$ orbital is the biggest orbital among all of the three and can allow the electrons to spread out more.
When we talk about hybrid orbitals and stability of charge on them, the electronegativity factor plays a more prominent role. The sp hybridised orbital is more electronegative than the others, owing to Bent’s rule which states - Greater the s character of a hybrid orbital, greater is its electronegativity.
Therefore, species having the negative charge on an sp hybridised atom tend to be less basic.
(You might be interested in knowing that, an sp hybridised carbon atom is actually more electronegative than a nitrogen atom.)