Head to the game folder and edit dosbox_arena.conf
with notepad: *fullresolution
should be set to your native screen resolution.
Or you can just set it to 0x0, which uses the desktop resolution. :P
Since I cannot post links for reasons uknown to me, here I post some attached screens of dosbox_arena.conf.
I have Direct3D renderer.
My fullresolution equals nothing. Changing it to my native screen resolution or 0x0 does not help, possibly it makes the border even wider.
Aspect is already true.
Thanks for quick responding.
Why didn't you try any of the renderers I suggested? Unless you're using a special build of DOSBox, Direct3D isn't even an option which might mean it's defaulting to surface which can't scale and would give exactly the result you described (higher resolution = bigger borders).
I find OpenGL to give the best results but you should try directdraw as well and see which is best for you.