This is a server configuration issue.
When resizing an image, it takes up memory on the server, the larger the image, the more memory is needed.
The server will have a setting in the php.ini to set the maximum amount of memory a script can use, wit the larger images, it receding the allowed memory usage.
you may be able to increase the allowed memory by adding the following code at line 1 of your page:
if that does not work, you'll need to contact the host.