Custom Java runtimes can be generated using jlink [1] which take a fraction of the space needed for a “full” JRE. I have some applications packaged like this and the Windows installer exe, generated using NSIS, is under 15 MB.
Sure, but then you have to add back all the Java libraries they then used ontop of it. I assume the developer also used a tool like this to cut it down, the issue of bundling the custom Java runtime was never the size, it's that java was never supposed to need to do it this way (not that that is the end user developers fault of course).