Hey OuterRim,
Morale is calculated by an algorithm that is unfortunately too complicated to explain here but the premise as you explained it, holds true. Positive and negative influnces have an impact on the calculation, they just don't translate one to one.
The expansion penalty does indeed rise based on the amount of provinces you own, as Cycle9 pointed out. What people most likely mean by "expanding slowly" is to build propaganda offices before conquering further provinces which slows down your advance.