Different aspect ratio for attract/coin-up and gameplay

[Roboman]

I notice that in some arcade games on MAME like Marble Madness and Xain'd Sleena the game's attract mode and coin-up screens fill my 4:3 screen, but when you start the actual gameplay, there are vertical black bars on either side.
Thought it might just be my MAME settings, but seems to be consistent in walkthroughs on YouTube.
What was the reason for this? Could the processors not handle rendering the gameplay full-screen or was there an extra bezel/border on the monitor?

[Mr. Peabody]

Less lines drawn equals less resources needed. In the case of Marble Madness (which does not change resolution, hence the aspect is the same, it could have been resources used as well as playfield look, the latter being a narrower area to keep track of. Coding scheme and such could've constrained the or promoted a certain geometry. Look up the game design for those. The creators may be living and available on the net.