it depends on the upload settings. If you set the upload to create multiple copies of the file, or if you set it to resize images it will take more memory than if it is just saving one copy of the file and not resizing.
you can increase the emory limit by using the following code at line 1 of the page with the form on it:
But It may not even be the memory limit, it may be another php setting like post_max_size
see this page for details on php settings that effect upload file size: