A good example is that most earth ponies use their bodies more, so their magic probably strengthens them. Whereas a pegasus uses it to fly as their wings are not really big enough for them to fly, yet they still do. On top of that some ponies also seem to have more magic than others the only reason I can think of for that is simply practice. From what I remember of the show most unicorns that are strong in magic grow up using it more than others.