AutoFit options may affect your table width when adding columns.
The AutoFit option currently associated with your table may affect your table width when adding a column. The AutoFit options are located in the Cell Size group of the Layout tab.
When you first create a table, it will automatically fit between the margins — that setting is called AutoFit to Window. When you use this setting, if you add a column before you add text, the size of the columns automatically adjusts to keep the table within the margins.
Another AutoFit option is AutoFit to Content, which changes column width based on what's inside the cells. If you don't have any text in the cells and choose this option, your cells will shrink to about a character width. If you begin adding text to a new table, and haven't chosen an AutoFit option, your table will adjust column widths to accommodate your content.
If you want your columns to remain the same width, you can use the Fixed Column Width option. It's great to use if you resize each column, which is what you'll learn about next.