Both internal factors (e.g., nonverbal intelligence) and external factors (e.g., input quantity) are claimed to affect the rate of children's vocabulary development. However, it remains an open question whether these variables work similarly on bilingual children's dual language learning. The current paper examined this issue on 805 Singapore children (4 years, 1 month to 5 years, 8 months) who are learning English (societal language) and an ethnic language (Mandarin/Malay/Tamil). Singapore is a bilingual society; however, there is an inclination for English use at home in recent years, resulting in a discrepancy of input between English and ethnic languages in many families. In this study, internal and external factors were examined comprehensively with standardized tests and a parental questionnaire. Regression analysis was used to address the questions. There were statistically significant differences in language input quantity, quality, and output between English and ethnic language learning environments. Singapore children are learning English in an input-rich setting while learning their ethnic language in a comparatively input-poor setting. Multiple regressions revealed that while both sets of factors explained lexical knowledge in each language, the relative contribution is different for English and ethnic languages: internal factors explained more variance in English language vocabulary, whereas external factors were more important in explaining ethnic language knowledge. We attribute this difference to a threshold effect of external factors based on the critical mass hypothesis and call for special attention to learning context (input-rich vs. input-poor settings) for specific bilingual language studies.