All snakes have the ability to shrink dramatically after being captured. Venomous snakes also have the ability to actually change species after death or capture as well. I have personally witnessed this phenomenon many times.
I don't even know how many times I've gotten a call that goes like: "I just caught a rattler! It's gotta be 6 foot long and has a head the size of my fist! You gotta get over here and get this thing, quick!" So, I load up and go to collect the offending critter and find a 5-gallon bucket on the driveway with a piece of plywood on top and 4 or 5 concrete blocks holding the plywood down. I remove all of blocks and plywood only to find a 2-foot Gray Rat Snake in the bucket, scared out of it's wits and just begging me to get it out of there.
I'm sure that after it is released it returns to it's real species and monstrous size, but I've never witnessed that part.
I guess the other possibility would be that the Rat Snake somehow found it's way into the bucket and ate the Rattlesnake, but that doesn't seem likely.
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We do not inherit the Earth from our ancestors; we borrow it from our children. Ralph Waldo Emerson