It does not take someone possessing a masters degree in theology to realize that Catholic Bibles are thicker than Protestant bibles. The difference in size stems from the Catholic Bible containing 7 more books in its Old Testament canon but why is there a difference at all?
During the time of Jesus and the Apostles, the canon (the list of books) of Old Testament scriptures was very much in dispute. Some Jewish groups, like the Sadducees, only recognized the first 5 books of the Old Testament as scriptures while other groups recognized a wider list of books as scripture.
During this time a Greek translation known as the Septuagint was in wide circulation. This translation of the Old Testament contained the canon of scripture recognized today by Catholics. The Septuagint become very popular among the early Church, in fact, the overwhelming majority of citations in the New Testament of the Old are from the Septuagint.
As the popularity of the Septuagint spread among early Christians, its canon of Scripture became more widely accepted by the Church until finally the Church defined this canon of Old Testament and along with the New Testament scriptures in the late 4th and early 5th centuries.
Meanwhile, at the end of the 1st century and early in the second century, Jewish groups began to settle on a canon of Scripture that matches the Old Testament used by Protestants today. It was not until the Protestant reformation in the 16th century that Protestant Christians started to recognize this canon of scripture as opposed to the one that Early Christians decided on in the late 4th century.