ryans Olanzapine 1.25 - 2.5 mg has decreased my pupil diameter dramatically. Doesn't touch D2 receptors at 1.25 mg and hardly at 2.5 mg (antagonizing D2 receptors is the anti-psychotic action of these drugs).
The other thing that would work is very low dose risperidone (0.25 -0.5 mg). This might be the better option since risperidone doesn't block H1 or muscarinic acetylcholine significantly at that dose. However I wanted the mild sedative effect of olanzapine so I chose it instead.
Passive 5-HT2a agonism is probably the MOA of SSRI-induced mydriasis. Someone not taking an SSRI probably wouldn't get the same miotic effect of these drugs.
Second generation anti-psychotics in low doses are the most selective 5-HT2a antagonists available on the market. The other commonly available 5-HT2a antagonist is mirtazapine but it is insanely sedating at doses needed to significantly occupy that receptor. It's one of the most potent H1 receptor antagonists available.
For a natural supplement one could try a Feverfew extract standardized to 600 mcg parthenolide. Mygrafew is one such product. I haven't tried it myself.
I have a "Pupil Size" PubMed collection if people are interested.
Here is a recent study: https://www.ncbi.nlm.nih.gov/pubmed/28504010
Pupil diameter was significantly larger in patients receiving SSRIs for <6 months and ≥6 months than the control subjects (3.53 ± 0.71 mm, 3.48 ± 0.60 mm versus 3.11 ± 0.72 mm, p < 0.05) but this effect was independent from the duration of SSRI treatment... Selective serotonin reuptake inhibitors cause mydriasis which is persistent during the treatment...