I think it depends on the band. I was in a marching band once with 9 people - tuba, bone, trumpet, clarinet, alto sax (me), piccolo, and 3 percussion. No problem hearing the sax (or any other instrument for that matter...)
The hardest part was marching in 5/4, we were doing an arrangement of Woody Shaw's "Theme from Zoltan". We also did something from The Firebird Suite.
I wish college marching bands would do more creative arrangements instead of the usual bombastic movie music themes. Marching bands go way back, well before written history, I mean the Gregorians walked while chanting, didn't they? It's just like TV or radio - a certain sound makes money so everyone has to do the same thing. The 9 piece band had a summer gig, I believe we got a grant, perhaps supplemented by a musician's union grant (remember those?), the pay was crap, but it was fun and people seemed to enjoy it.