There are two different horizontal settings for arcade mvs - 320x240 304x240. Metal slug uses the latter, obviously.
With the neocd, there's no way to switch between them without somewhat of a hack, so we default to 320 so that it doesn't cut off any of the edges. I may look into adding some sort of switch or setting to see if I can do it.
With the different resolution, it stretches differently.
Normally the cropping is handled by overscan on the monitor.