On any modern marimba or vibraphone, the bars are what is called “graduated”. Put simple, this means that the bars of an instrument are at their largest, both in terms of width and length, at the low end, while getting progressively smaller as the pitch ascends. This is in contrast to glockenspiels or xylophones, or of course, piano.
For reference, here’s a picture of my left hand holding position to play an octave at the bottom end of a 4.3 octave marimba (A2 to A3).
And here’s my right hand holding the same interval at the top of the instrument (C6 to C7).
While it varies slightly between instrument makers, the low octave is an entire 7 inches wider than the top octave. If it’s not obvious, the smaller interval is remarkably more comfortable for my hand. Not only is the hold itself harder in the large interval, but it requires more work to make the adjustment to get there. I’ll go deeper into this in a different post, but the bigger the interval change, the more work required. Going from a 4th to a 5th, for example, is usually quite easy, while going from a 2nd to a 9th is a bit of work to say the least.