Slideshow in Photos 7 for Maverick does not fill the screen

Correct. That's the way it works. The software designer must have thought it looked good.

It's just a convenience feature that you don't have to use if you don't like it. You can open the album, click Edit to get large images, and then swipe through the album, zooming on any pics that don't fill the screen.

Personally, I find the black background (border) of the "play slideshow" feature to be an attractive display. In the eyes of the beholder, of course. Nor do I see anything "silly" about the slideshow project feature. Don't see where you are coming from there. It's a quick and easy way to get a good slideshow product and export it as a movie. There are more complicated and feature laden slideshow apps out there, I am sure. You can even create a custom slideshow with iMovie.

-- Rich

Photos scale the photo to the screen, pinning the largest size do there is nothing cropped off the other dimension.

If you want to get rid of the border crop the photo to the same size ratio as the screen so that what the largest side is pinned so is the smaller side. However, for portrait oriented photos that would remove most of the photo.

What theme are you using. Just ran a test with the Ken Burns theme and filled the screen with the entire shot before starting the Ken Burns effect. The classic and others are a bit messed up in my opinion.

Give the Apple app, Keynote, a try. You can do so much with it and then export as either a Keynote file or video file.

Agreed that manually swiping isn't quite the same as a full screen slide show.

You can give product feedback to Apple at this link:

Product Feedback - Apple

-- Rich