Here is the frame size history:
Frame size history of #<frame *Minibuf-1* 0x56bbf9520158>
x_create_frame_1 (5), TS=80x25~>1280x875, NS=80x25~>1296x875, IS=80x25~>1296x875, MS=32x70 IH IV
gui_figure_window_size (5), TS=1280x875~>1280x1260, TC=80x25~>80x36, NS=1296x875~>1296x1260, IS=1296x875~>1296x1260, MS=32x70 IH IV
scroll-bar-width (3), NS=1296x1260~>1328x1260, IS=1296x1260~>1328x1260, MS=160x175
scroll-bar-height (3), MS=160x175
menu-bar-lines (2), MS=160x175
x_create_frame_2 (0), MS=160x175
xg_frame_set_char_size, invisible, PS=1328x1260, XS=1328x1260, DS=1328x1260
xg_frame_set_char_size (5), MS=32x70 IH IV
x_make_frame_visible
MapNotify, not hidden & not iconified, PS=1328x1260, DS=1328x1260
ConfigureNotify, PS=1328x1260, XS=400x340, DS=1328x1260
xg_frame_resized, rejected, PS=1328x1260, XS=400x340, DS=1328x1260
tool-bar-lines (2), MS=160x175
xg_frame_set_char_size, visible, PS=1328x1260, XS=1328x1260, DS=1328x1260
ConfigureNotify, PS=1328x1260, XS=2560x1346, DS=1328x1260
xg_frame_resized, rejected, PS=1328x1260, XS=2560x1346, DS=1328x1260
ConfigureNotify, PS=1328x1260, XS=2560x1326, DS=1328x1260
xg_frame_resized, rejected, PS=1328x1260, XS=2560x1326, DS=1328x1260
set_window_configuration (4), MS=160x175 IH IV
Note that something quite different happens when I open the second frame: it ends up full screen, but the Emacs window only occupies the same space as the initial window (roughly half the screen wide, and a little shorter than the screen).