I am wondering what house in your natal chart is ruled by Jupiter, etc.
It does seem rather odd when only considering Jupiter that it would suggest weight loss. But, when you think about what midpoints DO, it makes more sense. In Don McBroom's book Midpoints, which I truly loved reading, one gets a sense of how the planet in the middle is collecting the energy from the two on either side and then blending them together and then expressing them in a way that only the middle planet knows how to do. Jupiter is the planet of exaggeration, the planet of overdoing things. So, in that sense, it would not necessarily be expansion, but the exaggeration of the blend.
A Saturn and Neptune blend is possibly a despairing pair, but also very artistic: the "starving artist", or someone "down in their cups". A serious ascetic, maybe. I think an exaggeration could be dangerous, but Jupiter doesn't seem to be able to do anything joylessly. Like the Sun, Jupiter is dramatic. His goal is to show off a little, perhaps preach to the crowd. I'd say your Saturn-Neptune pair is lucky to have him there.