For the most part, I don't really notice a HUGE difference in my lashes when I switch between several mascaras that I like. However, there's obviously going to be the mascaras that suck, but I can never pinpoint why it is. Big brushes have both failed and worked on me. Same goes for small ones. And the same with fibre mascaras. You just never know...the combination of factors makes it so hard to tell (at least for me).
To me though, formula is always more important than the brush, because my biggest pet peeve (not just for mascara, but for ALL makeup) is smudgy mascara. If a mascara smudges on me I am NEVER, EVER using it again. I would seriously rather have a mascara be somewhat clumpy or make my lashes droop than use a fantastic mascara that smudges.