When you nest an existing table, it adjusts to fit the host cell. If the size of your table does not adjust correctly, apply the AutoFit option AutoFit to Window.
To nest an existing table inside a host table, simply copy or cut the table as you would to copy or move it anywhere in your document. When you place your insertion point inside a host table cell and paste, the table will be placed as a nested table.
However, depending upon the position of the cell where you are placing the table, Word might append this table to the host rather than nesting it. To avoid this and ensure that your pasted table is properly nested, type a space in the host cell before pasting the nested table. That space will turn into an extra paragraph mark above the nested table when you paste the table, but you can simply delete that paragraph mark if it is unwanted.
Note Apply the AutoFit to Window AutoFit option in your nested table so that it adjusts to fit the width of the host cell.
However, if you move a nested table to the body of the document and want it to retain its width, apply the AutoFit setting Fixed Column Width before copying the nested table out of its host cell.