Suppose you want to make a 2 gallon (approx 8 litre) batch of a 1.050 ale. You might need 0.75 million cells per millilitre per degree plato. So 750,000 x 8,000 x 12.5 = 75 billion cells. A smack-pack has 100 billion cells. So if it was produced the same day you want to pitch it, you are over pitching. So make 2.5 gallons (10 litres) instead of 2 gallons. Or what if your smack pack is a month old? Now you only have 76 billion cells anyway, which is perfect for your 2 gallons of 1.050 ale.
So having "too much" yeast in a Wyeast pack for small batches is unlikely to be a problem. Much more likely is that you weren't using optimal amounts of yeast for your 5 gallon batches--which is why most serious brewers use starters, dry yeast, or slurry.