> Assume FP64 units are ~2-4x bigger.

I'm not a hardware guy, but an explanation I've seen from someone who is says that it's not much extra hardware to add to a 2×f32 FMA unit the capability to do 1×f64. You already have all of the per-bit logic, you mostly just need to add an extra control line to make a few carries propagate. So the size overhead of adding FP64 to the SIMD units is more like 10-50%, not 100-300%.