The books of the New Testament are not arranged in chronological order. If they were, the epistles of James, Galatians, and First Thessalonians would be found immediately after Malachi.
The arrangement of the New Testament books has been given thoughtful consideration. The first four books—Matthew to John—focus on the person of the gospel. The fifth book—Acts—details the preaching of the gospel and the responses which result.
The third section—the epistles (Romans to Jude) is arranged quite intentionally. First, there are the epistles of Paul which were written to churches of Christ (Romans to 2 Thessalonians) and are placed by length (roughly from the longest to the shortest). Next there are the epistles of Paul which were written to individual Christians (1 Timothy to Philemon) and are placed by length (i.e., longest to shortest). Finally, there are the rest of the epistles (Hebrews to Jude); these are arranged according to author and length.
Because of its author (Paul), its recipients (the church in Rome), and its length (16 chapters), Romans has been placed immediately after Acts. But the serendipity of this arrangement is worthy of notice: the epistle to the Romans focuses on the power of the gospel.
Within the first six books of the New Testament, the person, the preaching, and the power of the gospel of Jesus Christ are presented to its reader.