As you expand, you have to balance out your resources to prevent shortages. Provinces you conquer will have low morale, so you won't have the full benefit of their resources until morale goes up. If you're facing a shortage because of expansion it will because newer provinces won't produce enough to offset the expansion...
There's also a military cost. You must leave a garrison in a newly conquered province until morale reaches 34%. That takes about 2-3 days if you only have one SEVERE shortage. The required garrison size to prevent a rebellion is available in the province view screen. As morale rises, you can siphon off the garrison for more expansion.
As you acquire new provinces the distance from the capital, measured in the time it takes to walk to the capital, determines how quickly morale improves. Railroads will help decrease the distance penalty.
Lastly, a centrally located capital is ideal for morale and defense reasons.