"The emscripten version is even smaller than the native version after compression, impossabru! I actually didn’t expect this myself (last time I did such comparisons the emscripten versions came out about 10..20% bigger)"

This is because JavaScript is alpha-numeric and is easier to compress than binary. There are a relatively 'small' number of characters in the alpha-numeric range, so they can be mapped together easier than the wider range of binary data.