Why are some JPEG files of the same scene much larger than others?
My guess is on autofocus or aperture briefly changing in a manner where the resulting image contains more high-frequency elements.

For example, if the focus moves from a smooth object to a textured object (like the smooth hippo to the cloth drape) in a manner where the latter's detail texture surfaces, JPEG will tend to take quite a larger size.

As someone else already stated: it seems like a good idea to check the image EXIF data for changes in core parameters like aperture and focus distance.  For such a striking difference in image size, it is quite likely that some fundamental parameter is different in the camera's opinion.