Political geography is different from political science in that it is a much more limited area of study. Political geography, like political science, is concerned with the study of politics. However, political geography mainly looks at the connections between politics and physical space. Because of this, it does not pay attention to many of the issues that political scientists look at.
Political geography is defined in the link below as “the study of the spaces of politics and the politics of space.” In a human geography book I have (Human Geography: People, Place, and Culture 9th Edition by Erin H. Fouberg, et. al.) we are told (p. 240) that “political geographers study the spatial manifestations of political processes.” In other words, political geography is only concerned with politics as it relates to space. Political geography studies how states are created and how they come to control a given area of land. Political geography studies why certain political systems are distributed in given ways across the face of the Earth. Political geography studies how countries divide up political power between their various regions. All of these things have to do with politics and space.
By contrast, political science goes far beyond the study of politics as it relates to space. Political scientists study all aspects of politics. For example, a political science department will offer courses in political philosophy. It will offer courses in how public opinion is formed. Political scientists will study the ways in which interest groups influence members of Congress or the ways in which social movements behave in their efforts to achieve their goals. All of these are important aspects of politics, but they have very little to do with politics and physical space. This means that they are part of the study of political science but not part of the study of political geography.
In short, political geography is a much more limited area of study because it studies only those aspects of politics that are in some way related to space. Political science is a field that studies all aspects of politics.