In this study, in order to analyze the urban heat island effect of large and small cities according to city size, soil and air temperature were collected for 53 years from 1970 to 2022. Based on the temperature data, a database of the minimum, maximum, average temperature, and temperature increase rate by underground depth was constructed and the impact of the city size on the heat island phenomenon was identified. As a result of the analysis, the heat island effect was confirmed not only in the air but also in the soil temperature in urban areas, and the short-term and long-term changes in soil temperature were greatly affected by air temperature. In addition, the rate of temperature increase in large cities during both winter and summer was greater than that of small cities, and the difference in the rate of increase in winter was larger than in summer, confirming that the heat island effect in large cities was worsening compared to small cities.