The main aims of this study were to investigate the relationship between the lexical size and the emergence of morphological and syntactic markers in toddlers between the ages of 16 and 30 months and to compare these results between Galician and European Portuguese. Parents of 3012 Portuguese toddlers and those of 1081 Galician toddlers completed the MacArthur–Bates Communicative Development Inventory: Words and Sentences. The results indicated that the number of words, the ability to combine words and the number of different morphemes produced increased with age. The ability to combine words was used as an indicator of syntactic development; this ability was also associated with the toddlers’ lexical size. In both samples, gender morphemes seemed to be the first to have their production generalized, followed by the plural and the past participle. The production of gender morphemes was accompanied by a small lexical size, whereas the imperfect tense and the person mark onset were associated with large lexical sizes. The implications of these results for charting the continuity between lexical, morphological and syntactical development are discussed.