Some career sites have a size limit for uploading documents to an application. Typically this will be about 3MB. Documents that are larger than this cannot be uploaded, as they are difficult to manage for the employer's recruitment team who will process your job application.
To reduce the size of your document, edit it and remove any large elements that take up space. Typically these will be large or complex images or embedded objects such as sound files or presentations. Sometimes also, older versions of programs such as Microsoft Word may waste space inside an apparently simple document: you may be able to overcome this by cutting and pasting the entire contents of your resume into a new document.