Hi! This is a great question. When you are formatting your campaign the default setting is for the toolbar to be set on paragraph formatting. This means that if you set one line of text to a large font and then hit return the editor is going to add a reasonable amount of space below that large text.
The workaround is to set the toolbar to one of the header options when you are formatting your header or subhead text.
Choosing Header 1 seems to have the least amount of space below the header.
Please let us know if there is anything else we can do for you.