Due to the different design requirements of Plasma Active compared to Plasma on the normal KDE desktop, a lot of KWin can be removed at build-time. With KDE SC 4.7, build options were introduced for KWin to remove some features unused by Plasma Active, such as for the X Render back-end since the Active version only targets OpenGL / OpenGL ES. For KDE SC 4.8, much more work is being done in this area to remove unnecessary code from being built and loaded by Plasma Active.
With the next KWin, there's now a build-time option to strip out the window manager's support for window decorations, configuration modules, reduced number of effects, no screen-edge interaction, etc. This is leading to a significant reduction in size for KWin when targeting these mobile / unconventional form factors.
Read more in Martin Gräßlin, the lead developer of KWin. For those interested in learning more about Plasma Active, visit this KDE.org area.