I have worked with the project code shared by you and like to share that its not an issue. Actually, in Aspose.Slides for .NET there 576 pixels per inch. The usual slide size is 10'' width and 7.5'' height that makes 5760 x 4320. However, in case of PPTX its actually scaled down to match the dimensions so it gives 720 x 540. Please visit this thread response for your further knowledge. Hope it will clear the situation for you.
Thanks and Regards,