As many have noticed, the size of Opera Mobile for Android is more than 20 MB. But did you know that the real size is actually around 10 MB?
How can that be? …
The short answer is that there are limitations in the way Android and Android Market work. Data is duplicated because of the packaging system, which increases the total size. The result is that there will always be a significant amount of things you don't actually need on your phone. And there's no way for Android Market applications to do clever things by detecting what could leave out from the package.
The Opera Mobile blog has a lengthy article with more details on the issue, so please read more about it there if you are interested.
The article also addresses Opera's size compared to other Android browsers, and points out that unlike the others, Opera comes with its own browser engine (Presto). Presto is a central part of Opera, and allows the browser to be uniquely optimized compared to browsers that merely use the existing engine on the phone.
Finally, the article tells advanced users who want to save some space how they can download a special package. Since this causes some limitations, you should really only do it if you know what you are getting yourself into.
Of course, there's always Opera Mini, which makes you a lot faster on mobile networks (and sometimes on wifi as well).