Third Cranial Nerve (CN III) Palsy: This is the most likely diagnosis because the third cranial nerve controls pupillary constriction. A palsy of this nerve can result from various causes such as aneurysms, diabetes, or trauma, leading to decreased pupillary constriction on the affected side.

Adie's Pupil (Tonic Pupil): Characterized by a dilated pupil that reacts poorly to light but has a more pronounced response to near vision. It's often associated with diminished or absent deep tendon reflexes.
Horner's Syndrome: Results from disruption of the sympathetic nerves supplying the eye, leading to miosis (pupillary constriction), ptosis, and anhidrosis. However, in the context of decreased pupillary constriction, it might seem counterintuitive, but the key feature here is the lack of dilation, not constriction, due to the sympathetic disruption.
Pharmacological Mydriasis: Certain medications or substances can cause pupil dilation, leading to decreased pupillary constriction.

Aneurysm of the Posterior Communicating Artery: Although less common, an aneurysm compressing the third cranial nerve can lead to pupillary dysfunction. Missing this diagnosis could be catastrophic due to the risk of aneurysmal rupture.
Pituitary Apoplexy: A medical emergency where a pituitary tumor undergoes sudden hemorrhage or infarction, potentially compressing nearby structures including the third cranial nerve, leading to acute pupillary dysfunction.

Syphilis (Tabes Dorsalis): A late manifestation of neurosyphilis that can affect the pupils, leading to irregularities in pupillary reaction, including decreased constriction.
Autonomic Dysfunction: Certain conditions affecting the autonomic nervous system, such as diabetes or amyloidosis, can lead to pupillary abnormalities, including decreased constriction, though these are less common presentations.