From
here...
".xap" files (pronounced "zap") use the standard .zip compression algorithm to minimize client download size. A "hello world" .NET Silverlight application (built using VB or C#) is about 4KB in size.
If a "hello world" takes 4KB already, what a
real application would take? A "hello world" in AS3 seems to be 400bytes btw, a 10%.
Lots to do
M$, lots to do...