How does having a bigger engine placed slightly more forward and higher impact the plane in pitch?
This is an "I think" rather than "I know" answer so if you disagree please feel free to explain why. 

In normal straight and level flight it shouldn't have a significant impact. But at a high AoA, at least a significant portion of the engines are ahead of and crucially, above the CG. The nacelle will be producing both lift and drag the resultant effect of both forces, being above the CG, will be a pitch up moment. 

Even if this extra force by itself isn't enough to make the aircraft unstable in pitch, it will require the pilots to make a stronger nose-down command, but when you're so close to a stall you want that to be as easy as possible.