When a table is created, exported to HTML and then imported, the cells become taller. The reason for this behavior is that the table is by default with style TableGrid. When the table is exported to HTML, the style is preserved, however, upon import the style properties are set as local and the style itself is lost. The style by default defines LineSpacing="1" and SpacingAfter="0" for the paragraphs in the table and these are not copied as local. 

This means that if there is a style set to the table that defines paragraph properties as well, these properties will be set.