Harry,
The reason the page expands is because of the "discount save_percent_box"
To kill it, in product.tpl, FIND:
Code:
<span id="save_percent_box">{if $product.taxed_price >= $product.list_price}{/if}
{if $product.taxed_price gt 0 and $product.list_price gt 0}
<table align="center" width="60"><tr><td class="SaveMoneyLabel">
{math equation="100-(price/lprice)*100" price=$product.taxed_price lprice=$product.list_price format="%3.0f" assign=discount}
<span id="save_percent">{ $discount }
</span>%
</td>
</tr>
</table>
</span>{/if}
Comment out most of this. I think you can comment all of it out. You may have a table formating issue... but maybe not if it's all commented out. product.tpl is a mess of nested tables. But this is definitely the code that's displaying the discount banner, and if removed, it will fix your issue.
Hope this helps.
Jeremy