If you wish to downsize images, use the compress_news_images setting in your recipes.
Code:
#: Set this to False to ignore all scaling and compression parameters and
#: pass images through unmodified. If True and the other compression
#: parameters are left at their default values, jpeg images will be scaled to fit
#: in the screen dimensions set by the output profile and compressed to size at
#: most (w * h)/16 where w x h are the scaled image dimensions.
compress_news_images = False
#: The factor used when auto compressing jpeg images. If set to None,
#: auto compression is disabled. Otherwise, the images will be reduced in size to
#: (w * h)/compress_news_images_auto_size bytes if possible by reducing
#: the quality level, where w x h are the image dimensions in pixels.
#: The minimum jpeg quality will be 5/100 so it is possible this constraint
#: will not be met. This parameter can be overridden by the parameter
#: compress_news_images_max_size which provides a fixed maximum size for images.
#: Note that if you enable scale_news_images_to_device then the image will
#: first be scaled and then its quality lowered until its size is less than
#: (w * h)/factor where w and h are now the *scaled* image dimensions. In
#: other words, this compression happens after scaling.
compress_news_images_auto_size = 16
#: Set jpeg quality so images do not exceed the size given (in KBytes).
#: If set, this parameter overrides auto compression via compress_news_images_auto_size.
#: The minimum jpeg quality will be 5/100 so it is possible this constraint
#: will not be met.
compress_news_images_max_size = None
#: Rescale images to fit in the device screen dimensions set by the output profile.
#: Ignored if no output profile is set.
scale_news_images_to_device = True
#: Maximum dimensions (w,h) to scale images to. If scale_news_images_to_device is True
#: this is set to the device screen dimensions set by the output profile unless
#: there is no profile set, in which case it is left at whatever value it has been
#: assigned (default None).
scale_news_images = None