Yes I read that as well. Bilingual speakers tended to have smaller vocabularies in both languages then single language speakers from what I read. Neural pruning in a nutshell. Basically an infant's brain has more neurons (brain cells) than it actually needs. As the infant grows, its brain eliminates all neuron that are not in use. Think pruning an overgrown shrub. Many of the neurons that are eliminated are the ones designed to recognize phonemes (foundational sounds of language). Different languages use different phonemes and an infant's brain can learn any phoneme it encounters. However, when these phoneme recognizing neurons are eliminated, that person can no longer hear or distinguish that particular sound. Hence, accents and difficulty learning a new language later in life. Now that it has been mentioned, you will probably recognize this effect when you engage with others who speak a different language, that they did not learn as a child.
When encountering someone trying to learn a phrase of another language, how many times have you heard a person say "I'm saying it exactly like you're saying it!" After being told to try again multiple times? Another example: before English was taught around the world many Asian base language speakers had difficulty distinguishing the difference between "L" and "R" sounds so would sometimes mix them up. L and R sounds are phonemes that arent used in these Asian based languages. Because of neural pruning these speakers had a difficult time hearing these sounds when spoken by others and difficulty making the sounds themselves.

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