Either there might be some media queries defined within your stylesheet like
@media handheld, only screen and (max-width: 1024px)
{
//Style goes here
}
Which is a style defined for screens having maximum width of 1024px.
Another possibility is, styles may be defined fully in percentage. So that the styles are changed according to the window size.
Again there might be some scripts used in your code to resize. All depends on the code that you are using. There are variety of methods which fits the view according to the screen size.