It has to do with the drugs mechanism of action. Drugs that cause pupil dilation are mydriatic.
This can be induced by mydriatic (pupil dilating) agents, typically those with adrenergic or anticholinergic activity. Selective serotonin reuptake inhibitors, including paroxetine, have comparatively weak anticholinergic and adrenergic activity,1 although stimulation with serotonin itself induces mydriasis and raises intraocular pressure in some animal studies,2 and an effect on serotonin receptors should be considered as a possible alternative mechanism. In patients mydriasis is associated with administration of fenfluramine, which induces release of serotonin,3 and with the selective serotonin reuptake inhibitor indalpine.4 Mydriasis has been described with paroxetine itself, particularly in overdose,5 although this could be due to enhanced anticholinergic action in high dose.