The common standard of the traditional stock photography industry is the size in megabytes (MB). The value of an image in MB sounds more appealing to the customer. Nothing more tricky than that!
The common standard for (educated) designers is the size in pixels (MP) that leads to discussions about dpi, ppi etc. It is this size that tells them how large an image can be printed.
The traditional industry uses this parameter as a trick. Requiring photographers to upsample their work (enlarging image size with a software) without any concerns about the final image (which usually ends up fuzzy and blurry) is simply wrong. Because not all images can be upsampled properly and the designer can do that easily.
The size in Mb ends up very high taking huge amounts of storage and upload/download bandwidth. Think about 10MB added this way without any gain for the quality and multiply them with several hundreds of downloads. You end up wasting GBs of data and server resources for a single file! Talking about climate change and green energy programs?!
Then the designer has to downsample the image in order to use it. Because he doesn't know the original size (unless he has an EXIF data to identify the camera and find its original resolution) he is left in the dark, having to guess how much to decrease the image. Each upsize/downsize of an image brings blur therefore a sharpen filter has to be used. Which leads to more artifacts.
The size in MB (megabytes) is the physical size of the file on your disk. It has nothing to do with the size in pixels. Hilarious, because you want to estimate what you see on the screen or in print, not how much it takes on your hard disk.
The size in MB can be higher for a small fall foliage image than for a large image of fluffy clouds in the sky.
We don't forbid upsampling of images. If the camera was low in MP you can bring the image to a better level by enlarging it a little bit. You should not exceed 10-20% though, although in some cases I've seen images at a double size looking good. But always check the final result at 100% and see how the focus areas have changed!
Photoshop does a great job at this and there are software programs even more powerful like Genuine Fractals Print Pro
and PhotoZoom Pro 2.1
While upsampling images is not bad, asking photographers to submit purposefully upsampled images IS bad, unethical and a waste of resources!