1
Your browser resizes first
Before a file leaves your computer, your browser scales it to 2560 px on the long side. That's why a session uploads faster than the file sizes on your drive suggest, and why it uses less of your plan's storage.
2
The server adds compression
On the server, the saved copy is checked again, resized if needed, and then compressed. That copy is what gets the watermark and becomes the preview your client sees.
3
What this means in practice
- a selection gallery looks good on any screen, including a large monitor,
- it takes a fraction of the space full files would,
- it isn't suitable for printing or archiving, and that's not what it's for,
- you edit the photos your client picks from your own originals.
4
When there's no resizing
In a delivery gallery, the file goes up at full resolution, with no scaling on either side. Accounts that bought the original files archive add-on before it was retired work the same way. Uploads take longer then and use correspondingly more space.