Its because you need to adjust the paper size format., like increasing or decreasing the page height and width

<field name="page_height">150</field>   

 <field name="page_width">60</field>

<field name="margin_top">5</field>   

<field name="margin_bottom">5</field>

OR

You can check it by : Settings -> Technical -> Reports -> Paperformat:

Adjust above mensioned fields and try printing reports

Hope it will work!!