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Carnivorous plants trap prey because they live in nutrient deficient substrates. The primary nutrient in low supply is nitrogen. Nitrogen is a limiting factor on growth size. So one reason for their small size is that eeking out nitrogen from prey is a slow process and provides only so much.

Another reason from the article, trapping larger prey would require more complicated traps, which would require more nitrogen.

“Carnivorous plants already grow in nutrient-poor habitats,” Sadowski says. The plants have evolved their various trapping mechanisms to feed upon animals because they are unable to get all they need from the environments they grow in. Carnivory is a workaround to be able to grow where they are. The biomechanics of a trapping mechanism strong enough to hold bigger animals aside, a larger carnivorous plant would have to grow in a habitat with better soil to get so big, in which case there wouldn’t be much reason for them to be carnivorous at all.