Photoshop is not a file editor it save image file for your document. You can not show file sizes for a file that does not exist windows explorer is not god there is no PSD file backing the current document state you made changes. Photoshop message states if you save your current docment as a PSD file it would exceed 2GB it the case you must use large file format Photoshop native file format PSB. Any size information Photoshop shows is not any file size. Most Image file formats support compression. File size is unknown till the document is encoded for a particular file format.
Did you increase document size convert to 16bit mode or 32bit mode use image size add layers etc.