For instance, 256b.net demos aren't 256 bytes of assembly, they're 256 byte executables. If you download a demo, you can see that the ASM files, even with comments removed, are typically 1 or 2 kilobytes.
So now we have less functionality and worse performance in the same or half as much space, and that's still ignoring the fact that the mapping of source code bytes to actual instructions don't in any way resemble each other. If you included some measure of information density, you should probably be allowed at least another kilobyte of Javascript.
Then we get to functionality and performance. We all (hopefully) know the tradeoffs inherent in dynamic and JIT-compiled languages running in a browser that's trying its best to be an entire OS with all the intervening cruft between code and hardware that that implies. On the other hand, the 256b demos require DOSBox because they use extremely specific hacks, so should js1k demos be allowed to use one specific version of a browser, with (for instance) a known math and canvas object key order so it never has to refer to anything by name? Should it be allowed to use WebGL? The actual competition requires them to work in all "current browsers", including IE, but if you could get the WebGL syntax overhead to work in 1k, you could kick the crap out of the 256 byte demos...but what would that even mean?
In other words, it's an important consideration but this is a terrible comparison :)