Q: Double click in photos 2.0

When I double-click thumbnail of a photo in an album, the foto is usually displayed in such a way that in fills the window, without borders, like this (right corner) - photo has 1600x1200 pixels:

I have some photos where the double-click does not fill the window, but has a white frame around. Like this - photo has 879 x 585 pixels:

Is this what you mean? Than I have it too.

But I do not know if this worked differently before. I think it always showed the white border when the picture was rather small in pixel size.

Photos displays the image at full size, pixel wise.  If the Photos window is larger than the image size it will be displayed with the white border. 

However, all you need to do is move the slider at the upper left hand corder of the window to the right to increase the image size to fill the window:

Thank you OT. I thought it was supposed to maximize the photo within the available space.

I believe iPhoto did but with small images that produced a pixelated image.  Now if you want it larger (and pixelated to some degree)  it's a manual operation.