On the single product page, the main image will be half the width of the page container.
So, if you are using a wide ‘Container Width’ (see ‘Customize’ > ‘Layout Options’ > ‘Container Width’) then it may be that 450px is too small. We recommend using a main image width of around 900px or adjust the page container width to a smaller value if you don’t have large product images that you can use.