I&#x27;m pretty sure this is because internally, it has multiple copies of the assembly supporting different instruction sets. So it will have a lib with &lt; SSE3, and then another copy of the same code with the SSE4.x optimizations applied. That way you get the speed benefit of SSE4.x on machines that have it, but it can still fallback to SSE3-only if it doesn&#x27;t have that ISA.<p>The Microsoft compiler works similarly. This is why &quot;Release mode&quot; builds with optimization on produce bigger binaries than &quot;Debug mode&quot; builds.