Runtime sizes are not very important for established technologies. Once a technology is so widely distributed that a typical user would be surprised to not find it on a machine, the download costs are mostly ignored.
With newer technology, that is not the case. Even mainstream platforms like Java are not ubiquitous and with a minimum footprint of 95 MB, users often balk at downloading it just to use a small utility.
Miguel quotes the full installation of Mono at a hefty 78 MB. When other related libraries like Gnome# and Gtk# are included, it jumps to 250 MB. But these are full installations, what about the bare-bones minimum?
Mirco Bauer, who maintains the Mono package on Debian Linux, has split the Mono runtime in such a way that the minimal installation is a mere 7 MB. This is of course just the runtime itself and the necessary libraries for non-GUI, "Hello World" style applications.
He compares this to the minimum size of Python, which weighs in at 13.4MB on the hard drive. As for Java, Mirco Bauer says:
So and Java? Here it becomes a bit unfair as SUN's license is not allowing to ship parts of a Java runtime. So the Java package in a distribution is one package. 34.5MB to download and 95.2MB on the harddisk. Yes, that's alot, but thats the smallest install you can get when you want to run anything on it.
On a final note, Miguel is estimating the final size for Moonlight, the Linux implementation of Microsoft's Silverlight, will be approximately 8 to 9 MB on disk.