After watching some of my replays, I realized that when playing against other Terrans their economies jump significantly larger than mine even though I don't keep unspent resources long (average unspent about 200-300), and I think it's due to the use of Mules. Is this true? Do Mules significantly impact the initial economy size of Terrans? I realize that one Mule can produce about 250 minerals at no impact to supply requirements, and that the rate they pull in minerals is significant.

So, my basic question here is that if I'm not using Mules I really can't hope to compete economically with a Terran who is?