[Windows Phone 8 General] StevieBallz:
Interesting sidebit: in internal Google E-Mails that got published during the Oracle vs. Google trial over Java it was actually mentioned that using C# instead of Java would have been an option due to the fact that there are less licensing hassles attached to it's core library (which actually is standardized with ECMA) as compared to Java. They decided not to go that route as it would have taken a year to adapt Android and instead risk getting sued by Sun (which was later acquired by Oracle). So: yes, C# would have been just as good an option. Using something like Google Go wouldn't have simply because there was no developer community and it's a lot easier to get people working on your platform if they don't have to learn a new language first.

That aside: most likely the binary size isn't all that much relevant for how big the downloaded files are. And I won't even go into the fact that some of those Apps aren't written in Java on Android but use the NDK (at least Facebook and the Angry Birds games do on Android, most likeley the later do it on WP8 too).

So in the end it's most likely down to the embedded Audio and Graphics resources. As was already mentioned Android devices have to support a lot more resolutions which makes it likely that LowRes graphics are included as well to not tax slower devices with high-res graphics for no reason (given that you won't see the difference on LowRes displays). Another reason for this with regular Apps is that WP takes a chromeless-design-approach so you rarely have graphics included that serve as UI chrome.

Another reason might be that Microsoft put quite some effort into driving home the point that resources should not be included or used in a higher resolution than what they are intended to be displayed at. The reason was that it might have led to troubles with the memory-constrained Tango-devices which only have 256 MB of RAM. At least for high-profile developers that work together with Microsoft it's likely that those optimized their Apps for it.

Lastly and also already mentioned: third party libraries. Historically Microsoft has always packed a lot of functionality directly into it's system frameworks. So it's entirely possible that WP devs use third party libraries less often. Case in point: database functionality: many Android Apps use SQLite and include their own binaries for it. WP provides SQLServer CE which can simply be used by any App that needs it. This might change though as for W8/WP8-cross platform Apps Microsoft themselves suggest including SQLite given that there is no SQL CE Support for WinRT-Apps.

And for the finishing lines something on compilers and code size. Intels C++ compilers regularly produce bigger binaries because of optimization techniques like loop unrolling, etc. They also normally outperform competing compilers in performance benchmarks. But it's not that easy if you look not at a single App's performance but at the whole system. Having an App take up more memory means that other Apps will have to be terminated sooner to avoid an out-of-memory scenario and it is more taxing on the memory controller, which depending on the chipset used might lead to additional performance problems down the road (the Nvidia Tegra 3 is said to be severely limited by its memory controller). But especially with Managed Code like C# or Java the code size of the IL does not really mean too much in that regard as the code is compiled anyway before being executed. So the memory actually taken up during execution is a lot different from what gets downloaded.

A more interesting comparison though would be wether the WP8 compiled XAPs are smaller than their WP7 counterparts, given that WP8 does precompile the IL in the cloud. Might be interesting to see which of those is smaller.