Change the size of this amount ONLY

Hi,

In your CSS code, you must have done something like that:
.xxx { ... CSS styles... }
However, that xxx class name is used on both the product page and the cart.
So solution is to prefix your CSS class with another class only used on the product page in a HTML element parent of the area you're targeting.
For example, you could change your CSS to :
.hikashop_product_page .xxx { ... CSS styles... }
That way, it would work the same, but only on the product page.