I think to have found the cause of this behavior. It seems that the text is centered between the top line and the top edge of the button and in turn the button is at a fixed distance from the bottom edge of the image. The consequence of this is that if there are more lines of text on the page, the image of that slide is enlarged.
This is verifiable in the BizLight Pro demo: the first slide has two lines of text and the other slides have a single line of text. Consequently, the image of the first slide is enlarged.
Is there any elegant way to avoid this?