I had a discussion with the main KWin developer a few years ago, about a related subject: restoring windows to their exact geometry (size AND position). Communication isn't always 100% clear with KDE devs but if I understood him correctly he claimed that this was not possible reliably under X11 and thus (would not be) not implemented. My X11-fu was already rusty at the time and while I remember(ed) it to be a bit tricky I also knew I had managed to get it to work in one of my own applications. IIRC, there are X11 geometry functions that just indicate a hint to the WM, but there are also ways to set geometry that the WM is supposed to respect.
Qt also has a geometry save/restore that works (if the WM doesn't interfere).
In short, try with a different, more basic but still standards-compliant window manager or two. That won't solve your issue if you are somehow tied to KWin, but it does give you reference material if you decide to take this up on a more appropriate forum (or even file a bug report).
(For me it ultimately led me to dump KWin and start using XFWM4 with the Mint-X theme as my WM, mostly because it's much less memory hungry.)
