"Is it because initially the engine is not given enough of a chance to draw in enough air/fuel within the cylinder and so with too quick of an advance we would be igniting an un-balanced/inadequate mixture? "
I can see no connection between spark setting ( advance) and air / fuel flow to the cylinders. Cam timing and valve size can influence air / fuel flow to the cylinders, but not spark timing.
Concerning initial timing and vehicle weight,, there can be two vehicles with very simular weight and engine parameters that each need different ignition advance curves.
And that would be because the vehicles have different gear set ups. A low 1st gear in a manual tranny lets a heavy vehicle power away from a stop with less effort (load) on the engine.
To suggest an answer on why a heavy vehicle would want a fast advance curve,, actually this applys to any IC engine. What you want is the spark curve that is optimized for that engine-vehicle-gear setup. The right advance curve is what separates a responsive engine from a lazy one.
this video may help you understand the basics
https://www.youtube.com/watch?v=HE-J-xhh1hs
