This study assesses vocabulary size of university students in an EFL context and unfolds the vocabulary development processes of successful learners through learners' reflective accounts. The study used a mixed methodology to collect and analyze the data. The quantitative data were collected from 93 university students who major in English Language Teaching (ELT) through vocabulary size test and the qualitative data through interviews with 11 participants who received the highest point from the vocabulary size test. The result of the study indicated that the vocabulary size of university students ranges from mid-frequency (6,000) to low-frequency (14,000) word families and the increase in vocabulary size is not linear from first to last year. It appears that individual learner's extramural English exposure from childhood to university plays a crucial role in the increase in vocabulary size rather than formal schooling. Technology use in the form of playing computer games, using online sites, and watching TV series and movies facilitate the acquisition and retention of new vocabulary for these successful learners.