I’m trying out this slideshow feature to the right. I think that if you click on the picture you can then click on the small icon at the bottom right with the four arrows and this will make it play full size.
In case I am seeing something different to you, this is the icon that I’ve indicated below that you are looking for,
It doesn’t work quite as you said. The picture on the right is only half size but if you click on “Marchena’s Building” at the top and then on “Slideshow” it works.