No as they are being displayed 1:1. If they Zoomed to Fit the screen they would be pixelated, 2:1 or 3:1 zoom.
You could export them with Re-size set to enlarge them to a larger pixel dimension, then use the up-scaled image in the slideshow. Lightroom would do its best to re-sample, but image detail and clarity will always mirror the original small file image.
I saw a similar post for someone using images exported out of MS-Power Point that did not fill the slideshow window.
Thanks. That's what I did and it worked well. It's too bad that Adobe decide they know better than the user and refuse to zoom past 1:1 rather than letting the user decide if they are wiling to use the pixelated result. As it is, for my purposes, the image was more important than the quality.
It's also too bad that Adobe do not document this - it took me ages to track down that it was caused by the original resolution and not by some setting I had messed up.