Why do drugs cause big and small pupils?!

Drugs can cause miosis (constriction of the pupil) or mydriasis (widening of the pupil) depending on which drug you are taking:

The sphincter muscle has a parasympathetic innervation, and the dilator has a sympathetic innervation. In pupillary constriction induced by pilocarpine, not only is the sphincter nerve supply activated but that of the dilator is inhibited. The reverse is true, so control of pupil size is controlled by differences in contraction intensity of each muscle. --Drugs like opioids like morphine or its derivatives can cause miosis by enhancing parasympathetic innervation which causes the pupil to constrict.

Wikipedia
Pharmacist

When ever you are in a state of panic your pupils dilate in order to let in more light so you are able to see danger. When you are calm, your pupils are not stressed so they remain normal sized. When stimulated by something nonthreatening (like light or a bright picture), Your pupils will shrink to prevent damage from light.

Certain drugs can stimulate different defenses:
Hallucinogens can make you seem stressed, letting in less light helps so pupils shrink
Anesthetics make you calm, relaxing your eyes so your pupils are usually normal
Any type of drug like LSD etc. can bring you to feel in danger so your pupils widen, letting you see danger even though there isn't any there

hope this helped =^.^=

I know a lot about a lot

drugs like mushrooms and other hallucinogens destort all of your senses. anything with DMT in it will alter your vision, hearing, touch, taste, even smell. basically when you're hallucinating, a lot of signals are being crossed inside your brain and your eyes have to adapt accordingly to a different type of focusing than they're used to.