Butterflies within the same species naturally vary in size.
Natural (good) reasons for size variation:
1. Sex of the chrysalis:
~ Eastern Black Swallowtail female chrysalises are larger than male chrysalises.
~ Monarch male chrysalises are larger than female chrysalises.
2. Genetic reasons:
~ Just as with other animals, sometimes a gene will cause someone to be much smaller or much larger than other of the same species
Other reasons for size variation:
The caterpillar did not eat enough food.
~ Not enough food available
~ Aggressive caterpillars push passive caterpillars from the food
~ Temperature too cold or too hot for caterpillars to eat (they cannot regulate their temperature)
~ Caterpillar diseased, causing lack of appetite
~ Host plant, while acceptable, is not favored by the caterpillar (much like us being offered stale bread)
~ Food contaminated with other substances, changing flavor of food (sometime splashed or spilled or was sprayed on the food)
~ Tender leaves have been eaten and only old tough leaves remain on the plant