They may have beaten the average compiler in x86 code density, but compilers aren't normally optimising for size and, as someone who has been writing x86 Asm for a long time, I can safely say that compiler output in general, even at max size opts, is far from the theoretical limit for x86 --- there is plenty of room for improvement there. On the other hand, RISCs are far more constrained in size optimisation. For an example of what I mean by x86 having a very high limit on code density, see https://news.ycombinator.com/item?id=15720923