Do you mean the width of the cell or the height of the row?
Generally, this will be the result if the columns are not into view as you export the data.
RadGridView exports the respective header cells with the minimum width value (which is 20 by default) if they have not been measured by the layout mechanism at the time the exporting operation takes place. To get all the columns exported with their actual width, you should first assure that they have all been realized.
You can also refer to the Export FAQ article for more details.
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