Originally I am a C# .NET developer but I am also very comfortable with Java. So what should I consider when making this decision?
I'm the same as you - Originally a C# .NET developer (WinForms and XNA, mainly) who is currently working on Unity games.
I have not used Android Studio for much (just rebuilding some plugins that another team wrote).
Unity on Android works great and is my opinion easier to deal with than the other three mobile platforms we support (iOS, WP8 and Metro). Code size problems are not as bad as it is on iOS, but Android's primary download limitation (not counting the OBB) is only 50 megs.
For decision making purposes: If you want to just make a game, use Unity. You're not going to suddenly decide to stop using an engine one day - there are too many good ones available that save a ton of time. You won't need to worry about writing your own renderer, and if you decide to, you will have ideas from Unity to lean on.
kburkhart84 is right that Unity's runtime libraries are pretty large. Rough sizes (some are out of date since I have an older version of Unity here at home):
Android: libunity.so, libmono.so and libmain.so add up to roughly 14 megs.
iOS: Your code will be AOT compiled and statically linked to Unity's libs. Our game is HUGE and has a ~30 meg iOS executable (this includes our code so it's hard to tell how much is Unity and how much is us). This will get DRAMATICALLY worse once Apple starts enforcing their ARM AArch32/AArch64 combined fat binary mandate.
WP8/Metro: Several DLLs of different sizes and flavors (ARM/x86, debug/master) - varies between 7-12 megs or so. The current downside on these platforms is that the build process is horrifically Frankenstein'd together and will explode at the drop of a pin.
Textures will likely be your #1 size problem though for an ambitious game. We have a seriously massive codebase at work (we have about every single type of software system you can imagine included in our game), and while code size is pretty depressing, it STILL isn't as bad as textures.