To start with, you must remember that the average total cost is made up of the average variable cost and the average fixed cost. Variable costs are those that vary with the amount produced (for example, the cotton that goes into t-shirts) while fixed costs remain the same no matter how many are produced (for example, the sewing machines or the building in which they are produced).
When you are only producing a few t-shirts, for example, the fixed costs make up most of the average total cost. You've paid a lot for the machines and the building, but you're only making a few shirts so you don't have to pay much for the cotton.
But now you start working at 100% capacity. You're buying lots of cotton and paying your workers lots of money. But the fixed costs have remained the same. Because of that, the variable costs are coming to be a much greater part of your total costs.
So -- this happens because as you make more of a product your fixed costs become lower in comparison to your variable costs.
Average variable cost (AVC) and average total cost (ATC) curves get closer to each other as the total output (Q) increases only when the cost follows a definite pattern as the Q is varied.
Typically the total cost of production can be divided in two components, fixed cost (FC) and variable cost (VC). The FC remains same irrespective of the output, while VC varies directly in proportion to the output. For cost of this type the average variable cost and average total cost are given by following equations.
AVC = (QxVC)/Q = VC
ATC = [FC + (QxVC)]/Q = FC/Q + V
As can be seen from other the difference between AVC and ATC is equal to FC/Q, which becomes smaller as Q (output) increases . This means AVC comes closer to ATC as output increases. In other words Average variable cost curve and average total cost curve get closer to each other as the total output increases.