This is weird, Tachyon Shield
has made a point I agree with!
When Voyager was first starting, a friend of mine tried to explain it as being like a swing wing on modern fighter aircraft. That explanation doesn't make much sense though. Modern swing wing fighters move there wings because at different speeds they need different aerodynamic properties (wide wing=lots of lift at low speeds, great for take off and landing. Narrow wing=minimum drag, ideal for high speed).
Now Voyager's warp nacelles are permanently up at warp, whether it's warp 1 or 9.99. So it can't be that.
My personal speculation is that having a lower centre of mass coupled with a wider profile could actually increase manoeuvrability at impulse speeds.