Version:            (using KDE 4.2.90)
Compiler:          gcc 4.3.3 
OS:                Linux
Installed from:    Ubuntu Packages

After selecting new plasma desktop theme, panels change their size.
For example: this is an excerpt from plasma-desktop-appletsrc before changing theme:

[Containments][31]
activity=         
desktop=-1        
formfactor=2      
geometry=0,-84,1920,24
immutability=1        
location=4            
plugin=panel          
screen=0              
zvalue=150  

And after changing theme:
[Containments][31]
activity=         
desktop=-1        
formfactor=2      
geometry=0,-201,1920,27
immutability=1         
location=4             
plugin=panel           
screen=0               
zvalue=150

Height of panels always increases about 2-3 pixels after changing theme.
After switching theme like 50 times in a row panels take up almost entire desktop.

I use gnome-like panel layout (one panel is on-top, one is on bottom).
Lower panel contains application launcher, taskbar and pager (4x3 desktops).
Upper panel contains XBar, tray icon list, device notifier and digital clock.

Such behavior started happening around KDE 4.1 - KDE 4.2.