Opera Mini 4 is based on the same rendering engine as the Opera 9.5 desktop browser. The browser still has a very small install size, less than 100k, even though we've added all these cool new features.
Go and take a look at the giant list-o-features, then tell anybody still stuck in featurephone land to install it post-haste on their phones. Windows Mobile users - well - you'll need to use it in a Java Virtual Machine.
I think that Opera Mini's zoom implementation might actually have a slight leg up on the native Opera 8.65's version. That version of Opera is my default browser, btw, or it least it is when I'm not using an iPhone as my main brain.