There are several modes for dealing with the screen. One of them is clipped, but I don't recall how it decides were the clip works (stretch the image to the first side it hits and clip the other perhaps). Look in the Windows Video Options section:
-[no]hwstretch / -[no]hws
MAME uses the hardware stretching abilities of modern graphic cards to
scale the game image to the requested resolution. Depending on the
quality of your graphic card and its drivers, this may be a fractional,
antialiased scaling (nice) or an integer, blocky scaling (not so nice).
In any case, you can disable this stretching altogether and let MAME do
the scaling in software, which is probably slower, though. The default
is ON (-hwstretch). Note: Vector games may actually look better with
"-nohws".
-cleanstretch [option] / -cs [option]
Stretch the image to integer ratios only. This may leave a black border
around the image in fullscreen mode. The options are:
none
disable. This will cause artifacts when using
scanlines.
auto
let the blitter decide. The d3d module will select
the best option.
full
always stretch to integer ratios both horizontally
and vertically.
horizontal always stretch to integer ratios horizontally.
vertical
always stretch to integer ratios vertically.
The default is AUTO (-cleanstretch auto). This option requires -ddraw
or -direct3d.
-resolution wxh[xd] / -r wxh[xd]
Specifies an exact resolution to run in. In full screen mode, MAME will
try to use the specific resolution you request. The width (w) and
height (h) are required; the color depth (d) is optional. If omitted or
set to 0, MAME will determine the mode auomatically. For example,
-resolution 640x480 will force 640x480 resolution, but MAME is free to
choose the color depth. Similarly, -resolution 0x0x32 will force 32-bit
color depth, but allows MAME to choose the resolution. The string
"auto" is also supported, and is equivalent to 0x0x0. In window mode,
this resolution is used as a maximum size for the window. The default
is auto (-resolution auto). This option requires -ddraw for full screen
resolution switching.