> So I double-checked -- and both behaviors with the latest patch match
 > what the current (unpatched) emacs-29 does.

With one subtle difference: In the unpatched version the WM shrinks the
size.  In the patched version we do it ourselves and the WM does what we
are asking for.

   x_new_font old char size 18x36 new char size 21x45 text chars 102x36 old text pixels 1836x1296 new text pixels 2142x1620
   xg_wm_set_size_hint scale 2 char width 21 toolbar 0 vscroll 32 fringes 16 borders 0 base width 34 width inc 10
       char height 45 menubar 50 toolbar 82 hscroll 0 borders 0 base height 88 height inc 22
   xg_frame_set_char_size old native pixels 1884x1296 new native pixels 2190x1620 outer pixels 1094x858 outer rest 0x0
   xg_frame_resized old native pixels 1884x1296 new native pixels 2188x1584
   adjust_frame_size old native pixels 1884x1296 new native pixels 2188x1584 old text pixels 1836x1296 new text pixels 2140x1584 old text chars 102x36 new text chars 101x35

Here we round down the outer pixel sizes from 1095x876 to 1094x858 to
meet the WM requirements (outer rest is 0x0 in all your steps).  So
incidentally, I reverse-engineered the WM.  Now let's try to round up
instead.  I'm quite confident that this will make your frames grow.

martin