据说是增量升级，但是还是不是很懂，不是前后两个都经过编译链接的产物，可以简单直接做加减法吗？
这是android4.1(好像是这个版本)的新特性，跟百度无关。具体你可以搜索下。
Google I/O 2012 时就已经宣布对 android2.3+ 的 Google Play 实行增量升级（Smart app updates）
Smart app updates: For Android 2.3, Gingerbread devices and up, when
there is a new version of an app in Google Play, only the parts of the
app that changed are downloaded to users’ devices. On average, a smart
app update is a third the size of a full apk update. This means your
users save bandwidth and battery and the best part? You don’t have to
do a thing. This is automatically enabled for all apps downloaded from
Google Play.1
实现来说是通过 bsdiff 来进行差分，详细的分析可以参见 这篇 blog.