Most of the previous studies focused on lexical and syntactic features in adapted texts while little attention has been paid to the structural complexity from the perspective of the amount of information. In this study, we intend to examine the differences in structural complexity in adapted texts from the perspective of the amount of information at the lexical and grammatical levels. We also aim to identify the lexical and grammatical features that contribute most to such differences. The data used in this study were reading texts from Newsela, that is, reading materials for English learners under the educational system of K-12 in the United States. This study yields several findings of interest. First, the adapted texts differ in the structural complexity at both lexical and grammatical levels in terms of their amount of information. Second, the differences display a decreasing trend. That is, the differences become smaller when the level of the text increases. Third, function words and part-of-speech trigrams with a preposition(s) and/or a noun(s) contribute most to the differences. The findings provide significant implications for the adaptation of language materials such as reading texts and test materials.