When you crop an image (regardless of the app you use) the resulting cropped image will always be smaller than the original. The only way you could "maintain the 8.2 MB size" would be to resample the image (increase the pixel dimensions) after you have cropped it.
In Photos you may be able to do this during Export. In the File > Export dialog, click the arrow button to the right of where it says Photo Kind: It will expand the options and you can Customize an export size. However, I don't know if Photos resizes or resamples the image ... resampling is the only one of the two methods that would increase the file size.
This might work to give you a larger file size but keep in mind that resampling an image to a larger size can easily introduce artifacts.