Interesting... I've not noticed a size difference, but all our caterpillars are brought inside (an inclosed porch), where the temperatures would remain warmer throughout the night, and perhaps they keep feeding longer. For an experiment, I should put some caterpillars in a cage outside in our screen tent and compare their final size to same age caterpillars kept inside. In the tent, they will experience the cooler nights. By keeping a couple pairs of monarchs in the screen tent (where they breed and lay eggs) we can be sure of having livestock for the start of the school season in September.
We keep potted nectar plants (for food) and potted milkweed (for laying) in the tent. Actually I found many of my houseplants thrive in the tent... Good thing the caterpillars only eat milkweed! That is Brenda (to the right) with her friend Sylvia (another monarch addict) showing off the new tent. Our neighbours think it is for us, and think we are weird when we actually explain what it is for!