Although technically, from a physics point of view, narrow bars give you less leverage, you'll find that absolutely pure sprinters prefer a narrower bar to a wider one. You don't see track sprinters using wide bars.
From bikeradar:
I find that I sprint slower with wider bars. It's not the leverage, or lack thereof, that hurts me. It's that I can't rock the bike as quickly or forcefully with wider bars - I want to be able to snap the bike from side to side. I went as wide as 44s for regular use, with 42s being "wide" to me. I ran 40 cm bars for a long time, then moved to a 41. I plan on moving back to 40 cm bars. With all those bars I did long rides, 5-7 hours (to me that's long) with a max of 8-10 hours on some of them (mostly the 41s and 42s).
This is me on 42s (photo by Bob Griffith). It shows how narrow the bars are relative to my shoulders. I won this race, which I actually forgot about until I went looking for these pictures:
Road sprinters will use wider bars than track sprinters, although, again, there are some riders using 38 cm bars who by conventional wisdom ought to be on 42s or 44s.
Greipel, from Ridley's site, who has sized down to a 38 cm bar:
A few Lotto guys use narrow bars after Adam Hansen got them to try the bars. Greg Henderson, another big guy on Lotto, uses 38s (or 36s but I haven't seen that referenced in more than one place). Sieberg uses narrow bars too, as well as Roelandts.