Currently, there is no option to remove the white space in an ArcGIS StoryMap.
The white space in between the contents or slides is there by design. This feature is fixed and necessary as the story in ArcGIS StoryMaps is a scrolling page and a series of content designed to fit any screen size. When the tour block locks into place, the first row of places begins fading away if there is no space between the contents. The white space functions as a form of buffer to ensure readers have a good experience and do not miss any part of the tour.
Additionally, the extra white space in sidecar features in ArcGIS StoryMaps is purposeful. Sidecar features have the extra white space below text or images to allow the main image to clear and for the story to transition to the next slide. This white space cannot be altered or removed.