The number of Representatives each state gets is determined by its population, as determined by the decennial census. The Constitution requires that Representation be apportioned among the States according to their respective Numbers, counting the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed.
The 1929 Permanent Apportionment Act set the total number of Representatives at 435. This number has not been changed since then, despite the country's population growth. As a result, some states have gained Representatives while others have lost them, based on their population changes.
The states with the most Representatives are California (53), Texas (38), and New York (27). The states with the fewest Representatives are Alaska (1), Delaware (1), Montana (1), North Dakota (1), South Dakota (1), and Wyoming (1).
The number of Representatives is different because the population of each state is different.