To reproduce this you need at least one secondary monitor.

1. emacs -Q
2. Move the frame to the secondary monitor
3. M-: (modify-frame-parameters nil '((left . .5) (top . .5)))
   frame is moved to the main monitor
4. Move the frame to secondary monitor again
5. M-: (modify-frame-parameters nil '((width . .5) (height . .5)))
   frame size is half of the main monitor instead of the secondary one

In my case the 'display-monitor-attributes-list' function returns these
attributes:

(((geometry 0 0 1440 900) (workarea 0 25 1440 875) ...)
 ((geometry -254 900 1920 1080) (workarea -254 900 1920 1055) ...))

Attached patch make the frame position and size ratios calculations
related to the monitor on which the frame is.