Actually, and I'm sorry that this one will sound as if I am arguing my validity but I did need to point out that even those contradicting me about the dependency weight are linking and proving my comments. I probably should have been more clear and mentioned that POST install can be 1GB or more and even with that I was partially wrong. As pointed out by pinebanana after installing .net for x86 you will require 850mb of disk space. That's a whole hell of a lot of space just to get a tool to work especially if you need that just to launch the window.
64bit installs can take up to 2GB as pointed out, however it's unlikely you will use or need 64bit functionality in an editing tool. Long story short you have to have your user's have or install the .NET libraries which can vary depending on which install you use but in most cases range in the 850mb area. For anyone who remembers this install process it's quite time consuming even on a fast system. When all is said and done and your asking your users to install 850mb (or even upwards of 2gb!) just to use a map editor, which in actuality doesn't even use 20% of the stuff you installed... That's my point. When you get away from windows and you have the comparable sizes of Mono (post install) it's even less likely the end user already has it or that they would ever need it for anything besides your editor.
So java has similar sizes post install, true. Difference? Most if not all of your end users most likely already have it installed. Does your team use blender? They have java, do they use open office? They have java. Have they ever used anything from Oracle? They have java... There are some things to take into account such as which JRE is installed and what you are building against but in most cases it's safe to assume that your end user probably already has java. On non windows platforms it's even safer to bet they have it. This equates to less install time, less disk space getting used and overall just less garbage that your end user has to deal with just to use your one or two editors.
With that said I should note that I am a huge C# and .Net advocate, I am not attempting to talk anyone out of using C# (as long as it's for windows), however considering the question relates to cross platforming we're talking more about forcing non windows users to install things that emulate windows natural behavior in a foreign platform to use a tool. These extra mono libraries that you are asking your user to install have nothing to do with their day to day computing and until we start seeing more and more mono ports of C# programs in the main stream they aren't likely to ever see or use it again.