Photos

after exporting photos from the "Photos" application to, for example, a desk, their size is reduced (it cannot be exported in the original size)

1. What do you mean by "size"? Are you talking about the dimensions of the photo (length by breadth) or do you mean the "weight" of the file (measured in kb or mb)?
2. In either case you can export a duplicate of the original file, same dimensions, same weight of file by using the File -> Export -> Export Unmodified Original
3. In the export dialogue size refers to the dimensions of the photograph. Full size will yield the same dimensions as the original (assuming you haven't cropped it of course).
4. In the export dialogue Jpeg Quality is what has most affect on the weight of the file. The higher the quality chosen, then the less compression applied, and the larger file. Lower quality means more compression, means smaller file size.

If you drag and drop then you only get a preview of the image and that is reduced in both quality and size.

Not sure what you mean by the first comment. You can download edited photos immediately, or use the share icon (square with the up arrow) to email.

You can elect "small," "medium," "large," or original size in the export options menu when you select file > export. Or, you can export the unmodified original.

But when you edit a photo and then export it, it has less weight

What Quality are you exporting it at?

To explain: Jpeg is not a photo format. It's a compression format. Your camera takes a snap, saves it as a tiff or similar and then compresses it into a jpeg, to save space. Worth noting is that in most cameras you don't have a choice in the amount of compression used.

When you edit a photo in Photos (or similar apps) you never touch the original. A new version of the image is created based on your decisions taken when editing. When you go to export, a new file is created and, just like your camera, that is compressed into a Jpeg. However, in Photos you can choose the amount of compression. As stated above, the highest quality uses the least compression, and so you get the "heaviest" file. Therefore it is quite possible to export a larger file than you imported... not because you've added data, but because you've used less compression.

Experiment with the Jpeg quality settings and compare the results.

I cannot recreate your issue. I selected a photo, edited it (adjusted colors, etc.), clicked the share arrow, selected email, emailed to myself. Clicked send/receive on my email to force the sending and receiving. The image was in gmail in just a few seconds.