As an example of what I'm talking about I did some simple captures.
On my system - with a fresh install of Mame, and a bog standard Mr. Do Zip and artwork file - running in a window for convenience, here is what I see when I use the Bezel I like:
Obviously that's Photoshop beneath it. That's actual size, and the gameplay area is very small. It's authentic - but small. For me, too small to play.
If I go into Video Options and crop, I get:
Which clearly is no good, because the Bezel is gone.
What I prefer is a default of:
This fills up much more of the screen, but it's not quite right yet. For a 4:3 screen in standard orientation it would be full screen (Although yes, on a real deal Mr. Do machine it would have been oriented differently as 3:4 - but that's a luxury I don't have.)
Going one better - if I go into the properties of the game (as we all know, this can be set on a global, or game by game basis) and turn off SCREEN - AUTOSELECT ASPECT, I get this:
This - for me, and perhaps ONLY me, is optimal. I have what is, in effect, full screen coverage. I have a large. fun Bezel, and large game screen. BUT - it goes without saying - for the last two examples I have "Butchered" the artwork to fit my own template, which no doubt will drive some people mad. But hey, that's why Mame is so great, we can configure it the way we each feel is best for our setup.
I haven't posted any "Butchered Artwork" Zip's, as I think that would confuse things - but for the games I love I'll do it for my own use. It's a compromise, but as nice as the original Bezel is - and the Mr. Do one is fantastic - it's the GAME I'm playing, and I want that nice and big, basically filling the entire space top to bottom (based on a standard 16:9 orientation for the screen).
As I said, there's no way to truly "have your cake and eat it" when it comes to this - hence why Mame is so cool in letting us choose.
That's for vertical games such as Mr. Do. For horizontal games things are a little different, as it's easy to fill the entire screen on a 16:9 monitor. However, that stretches the game, and is actually too big of a playing area for me on a 52" screen. So in that case I fixed up a template that worked for me. That is, a template that gives me space to have a Bezel, but with a game play area that is large, but not too large.
Hope that makes sense.