The filters are just simple image transforms. Generally they're just specified as GL shaders, which means that even in the largest case, you're talking about a kilobyte or two. Considering that they're compressed (as all APKs are), you can divide that by half, if not more.
Think it's the filters. Some are purely an algorithm, but I assume other filters work by merging the photo with a template image. Maybe this has to be stored in high res in the app.
Google+ is a good example of something that is plain wrong in my opinion.
I uninstalled it months ago, so I can't tell if it has great improvements. But from what I recall, it can be easily compared to... the Facebook app, for example. What is the purpose of those 20-ish Mb of difference between the two?
You would think Google would be making great and efficient apps with their own platform.
I imagine that the Google+ app is so big because it contains all the resources (images, translated text, layouts, etc.) for both the tablet and cellphone versions. Also, it needs to include extra libraries to support some of the newer ICS features on older phones/OS versions.
They might also rely heavily on a non-java codebase, in which case they can't just use the built in libraries that most android apps can rely on.
Both Chrome and Firefox's mobile versions use a lot of space too. (Firefox is down to 20MB, but chrome is 50.)
And wow, apparently Google+ takes 30MB! Not sure what that app does that makes it larger than firefox...