The board software handles photos differently depending on the source. I think if it is an external source, they are automatically sized to fit better. If they are a local source, they keep the size they were saved to. How big they are will also depend on what device you are using to view the photos.
I have actually cropped and resized (smaller) the pictures so they display better and less data is required for a download. The earlier pictures were huge and these are less than half the size.