Thanks for your question about making the homepage larger in the Swifty theme for mobile devices. This is not normally something you can do using the native controls within WordPress or BoldGrid. You would either need to use custom CSS to format the image differently, or you can obtain the actual file being used by the theme and then edit the file directly with a graphics program. This would mean locating the file, downloading, editing it (in a graphics program) and then re-uploading the file to your WordPress site. WordPress generates the different file sizes and saves them in the wp-content/uploads folder. It may take a little searching to find the file, but you can edit the smaller sizes so that the image looks larger within the size constraints that are being used. I hope that helps to explain the issue. If you require further assistance, please provide the URL and we can look at the issue in more depth.
If you have any further questions or comments, please let us know.