Not all 4 octave marimbas have narrow bars. The school I teach has two marimbas, both of which are kinda undersized. The smaller of the two is a 60+ year old 3½ octave Deagan, whose bars are a fairly consistent narrow width. It doesn't seem to go low enough to need much graduation. (It's definitely a marimba, though.) The other is a 4 octave rosewood Musser from the 1970s, whose bars get noticeably wider as you go down the keyboard.
Of the mallets I've tried, I get the best sound from the Deagan by using Pro-Mark Evelyn Glennie cord-wound mallets. (
HERE is the medium version on Steve Weiss' web site.)