I don't know serverless, but recently-ish (few months ago) I was testing Java memory usage on a tiny program. Something like:

    public class Main {
        public static void main(String[] args) throws InterruptedException {
            for (int i = 0; i < 60; i++) {
                System.out.printf("%d\n", i);
                Thread.sleep(1000);
            }
        }
    }

After spamming as many flags as I could find (literally listing all possible flags and seeing what looked like a high number, and lowering it), I could only get this program to use 307MB VIRT and 40MB RES memory.

Unfortunately I didn't save the flags I used.

That's more or less on par with the equivalent Node.js program on my PC (Node.js actually uses a bit more RES memory, 44MB).

So I guess as long as you can (and want to) spam a lot of flags, it's feasible to lower memory usage.

Disclaimer: Not a Java dev.