Maybe the owner was feeding her constantly, but it's likely that, along with the cheesy PETCO-recommended setup, they may have been feeding her smaller meals than what would be preferred, which may explain why she's so small. An appropriately sized pre-killed or F/T rat once a week would do and hopefully she'll be back up to size.
Also, my personal advice, find a better name than Trouser. Trouser the snake is even more lame than Monty the python.