Why are my nepenthes ventrata's ptchers so small?

I have had a N.ventrata for 2 years now. It only started pitchering mid last summer and all of its pitchers are really small. I feed it blood worms every month and every time a new pitcher opens. It is grown in low humidity but i have adapted it. i grow it under a compact fluorescent light and it makes 2 pitchers every month but they are only about 2 inches each and aren't getting bigger. the plant itself is about 13 inches and is very young. i do not fertilize and i water with distilled. Are they only this small due to it being young or am i messing something up.