If I download a Java application via a JNLP file, I think it then downloads something else too since the JNLP file is tiny. How can I find out how large this download actually is?
To know the size of a Java program which has been already downloaded with the Jnlp, you can open the Java Cache Viewer:
In this viewer, you can find the size of each of the applications you installed via JNLP.
Also, to add to davr's answer, an extract from the JNLP file syntax:
If you don't have the "download" argument for the jar described, it will default to "eagerly", as described earlier. So you can be sure that all jars which don't contain "lazy" will be downloaded prior to launching the app. You can then estimate the download size with all of them. "Lazy" ones, though, can be loaded afterwards.
That's correct, the JNLP is just a small text file telling Java where to download the actual application files. Sorry I don't know how to determine the size in an easy way, besides opening up the JNLP file in notepad, and manually downloading the files to see how big they are.