It’s not that, it appears. It seems that the images won’t use more width if there is no more height (they will not make the ScrolledWindow overflow).
I want the images to keep aspect ratio and take full width, even if it means overflowing and needing a scrollbar.
Edit : This behavior seems to come from the RequestMode of the Picture widget which is GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH and what I need is the opposite, GTK_SIZE_REQUEST_WIDTH_FOR_HEIGHT (setting width defines the height).
Edit 2 : The previous edit is wrong, the current RequestMode is good according to the documentation :
Height-for-width means that a widget can change how much vertical space it needs, depending on the amount of horizontal space that it is given
Edit 3 : It seems that @melix99 had the same question in another topic and it was unanswered.