You should be able to write a very basic WebAssembly compiler in much less than half a megabyte. The WebAssembly format itself can be assumed to already be pre-optimized, and is expressed in terms of i32/i64/f32/f64 with an infinite register set and stack machine, with a very well structured control flow. You really don't need a smart compiler to lower this to semi-efficient code. The runtime in a non-JavaScript embedder then has to expose a series of syscalls, but that's not very big either.