That would lead to a small image size because you would just apply the .jar to a base Java install but it doesn't do anything for the container size because they all will have their own JVM copy.
I don't understand what you mean for the container size. If they share the same base layer it won't use more disk space to store than splitting it up, surely.
You're missing the fact that if they're all using the same layer (base image) for the JVM all the images will be deduplicated (well, CoW) so you could have a hundred different apps and they would only use the equivalent of a single image for the JVM (in terms of storage).
