5-10 times smaller for regular ones with it closer to 50 for separation kernels. That's not a fair comparison, though, as you need extra components to support the unikernel or just components on the kernel. A more fair comparison might be L4Re, a stripped GenodeOS, OKL4 w/ necessary software, or NOVA with necessary software.
L4Re was the first attempt I believe at an environment for apps on L4:
L4Re was the first attempt I believe at an environment for apps on L4:
https://os.inf.tu-dresden.de/L4Re/overview.html