It appears you are using Lightroom Classic. Would you mind confirming the version of Lightroom you are working on?
The file size of the image is determined by the file type, diensions resolution of the file. Jpeg is a compressed file format & it does not contain as much metadata as a RAW file does & hence the file size is way smaller. For more info, please check: https://helpx.adobe.com/lightroom-cc/how-to/raw-vs-jpeg.html
You seem to be exporting the files at the same dimensions as the RAW file at 300 ppi which should be adequate for most use cases. If you wish to increase the file size further, you can try increasing the dimensions or resolution of the image while exporting.