Regarding fluency, bilinguals and monolinguals often have often have similar total vocabulary sizes, but bilinguals tend to have a significantly smaller vocabulary in each language.[1, 2]  In other words, vocabulary size(L1, monolingual) ~= size(L1 + L2, bilingual), and size(L1, monolingual) &lt; size(L1, bilingual).<p>1.  Portocarrero et al. (2007).  <a href="http:&#x2F;&#x2F;www.sciencedirect.com&#x2F;science&#x2F;article&#x2F;pii&#x2F;S0887617707000224" rel="nofollow">http:&#x2F;&#x2F;www.sciencedirect.com&#x2F;science&#x2F;article&#x2F;pii&#x2F;S0887617707...</a><p>2.  Gatt et al. (2015).  <a href="http:&#x2F;&#x2F;ijb.sagepub.com&#x2F;content&#x2F;early&#x2F;2015&#x2F;03&#x2F;13&#x2F;1367006915572399.abstract" rel="nofollow">http:&#x2F;&#x2F;ijb.sagepub.com&#x2F;content&#x2F;early&#x2F;2015&#x2F;03&#x2F;13&#x2F;136700691557...</a>