It's hard to say why these regions are called this way, as that has never been directly explained in any formal document. There are however some theories which I find to be pretty convincing. It's by far easier to say more about Greater Poland's name (in Polish it's Wielkopolska, literally Great Poland) than about Lesser Poland (in Polish Małopolska, literally Small Poland). The expansion of the country of Polans (one of the Polish tribes living in Greater Poland, which later unified other Polish tribes and created the first Polish state) in the 10th century meant that the original tribal territory of Polans, initially referred to as Polonia, later began to be called Polonia Maior (as opposed to Polonia Minor - Lesser Poland). The first to use the name "Wielkopolska" (Polonia Maior) in the preserved documents was Maciej of Lubin, a Benedictine from the monastery in Lubiń and a notary of the Duke of Kalisz, Boleslaw the Pious, who mentioned the name "Wielkopolska" in a document of June 15, 1257. Interestingly, the names "Wielkopolska" and "Małopolska" are relatively young. Such terms for individual Polish districts as Mazovia, Silesia or Pomerania are much older. For centuries, the land on the Warta River was called Polania or Polonia. These names came from the definition of the territory of the tribe of Polans and only later were transferred to the entire country ruled by the Piasts (first dynasty that ruled Poland). Until the thirteenth century, the term Polonia was used to describe the entire country under the rule of kings and Piast dukes, even during the period of the division into districts, as well as the lands on the Warta River, with Poznań, Giecz, Gniezno and Kalisz. Sometimes, however, it was necessary to specify exactly which lands are concerned, apparently it was a necessity. In the document of Władysław Odonic, the Duke of Greater Poland from 1234, there was already a distinction between the Polonia in the sense of the country and the Polonia, as the lands around the Warta River.

All the sons of Bolesław the Wrymouth called themselves Dukes of Polonia, and it was only in the next generation that the names of individual districts appeared in the official titles of monarchs (Duke of Sandomierz, Kraków, etc.), only the Dukes of Greater Poland consistently called themselves Dukes (dux) of Polonia although sometimes they also sometimes used the titles of the dukes of Poznań, Kalisz or Gniezno. In a document issued by Duke Bolesław the Pious in 1257, when the latter, after the death of his brother Przemysł I, became the duke of the entire Greater Poland, the name Polonia Maior appeared for the first time to describe his sovereignty, which can be translated as "Greater Poland". At the same time, however, the duke was called in the old way, the duke of Poland (dux Polonie). In the documents of the nephew and successor of Bolesław the Pious, prince and later king of Przemysł II, the term Polonia Maior appears simultaneously with Polonia. The term Polonia Maior appeared in the title of the Piast dukes fighting for power in Greater Poland after the tragic death of Przemysł II. After the reunification of Poland, the name Polonia Maior did not appear in the title of Polish kings, because both the Greater Poland territories: Poznań and Kalisz-Gniezno were covered by the title of "Polish king" (rex Poloniae). The term Polonia Maior did not disappear, however, and appeared in the title of the general starosts of Greater Poland. This office was introduced at the beginning of the 14th century by the King of Bohemia and Poland, Wenceslaus II, and survived until the partitions of Poland. The starosts appointed by the king to manage Wielkopolska on his behalf were called generalis Poloniae capitaneus or capitaneus Poloniae Maioris. In the second half of the XIVth century, the name Polonia Maior, used interchangeably with Polonia Magna, meant only the province on the Warta River. At the same time, the former principalities of Kraków and Sandomierz came to be referred to as Polonia Minor from the beginning of the 15th century (Lesser Poland).

The name Wielkopolska also appeared in 1449. It was used by Świętosław of Wojcieszyn in the Polish translation of the statutes of Casimir the Great. However, a problem arose, because the term Wielkopolska was used to describe both the lands of the former principalities of Poznań and Kalisz-Gniezno, as well as the entire province of Greater Poland, including the provinces of Kujawskie, Mazowieckie, Łęczyca and Sieradz, which additionally had a common sejmik gathering in Koło.

But what exactly does the adjective Maior mean? Translated literally from Latin, it means "greater". Greater Poland, however, was not the largest province of the Kingdom of Poland, so where does this term come from? Some twentieth-century historians were inclined to believe that Greater Poland was an inaccurate translation of the Latin term Polonia Maior. According to the supporters of this hypothesis, the term Polonia Maior should be translated as Old Poland or Older Poland. This theory, however, soon collapsed. Some linguists recognized that the word maior means not only greater in terms of size, but also something dignified, senior in rank and dignity, the most important. In the Middle Ages, Wielka Street was marked out in Poznań, which was preceded by the Great Gate (which does not exist). In both cases, we are not dealing with the largest thoroughfare in the city, but the most important and main street in Poznań. The main altar in a church was often referred to as the "great altar", and the "great court" was the main center of earthly goods. To sum up, in the Piast tradition and in the opinion of the whole nation, Greater Poland was the most important and oldest district of Poland, which gave rise to the entire state. Perhaps the magnates of Greater Poland, dissatisfied with their removal from the affairs of the country in the fourteenth and fifteenth centuries, emphasized that they represented the main part of Poland and its cradle, hence the popularization of the term Wielkopolska in the late Middle Ages.

The name Małopolska (Latin: Polonia Minor) was first used in written sources in 1411, and it was confirmed only in 1493 in the Piotrków statute of King John I Albert (during the session of the bicameral general parliament in Piotrków in the years 1493–1496) in order to distinguish this part of the country from Greater Poland. The original meaning of this name is not certain - the words maior and minor can be read here in several contexts, not only the most obvious ones - greater - less, but also, for example, older - younger, extended - proper, as at that time the capital of Poland was Kraków, which is in Lesser Poland, and that part of the country was the most important to the Polish monarchs