In any program, when a new window is opened, the program has the control on the appearance of the new window (size and position).
But unfortunately, instead of opening the new window directly with these values, it uses the current KM window values (step 1). Then move the window to the correct position (step 2), and finally, modifies the size (step 3).
You can see a screenshot showing this behavior here: http://users.skynet.be/alternity/k-meleon/km-02/KM_popup_trail_big.jpg
And a more thorough explanation must be read here: http://users.skynet.be/alternity/k-meleon/km-02/#7
(in front of image 7)