When volume increases, total variable cost increases; when volume decreases, total variable cost decreases.
Total variable costs increase as volume increases because there is more activity that incurs variable expenses. The variable cost per unit remains constant with changes in activity. In other words, the cost per unit of activity.
When volume increases, fixed costs remain constant in total because they do not change with changes in activity. However, the fixed cost per unit decreases as the total activity level increases. This is because the fixed costs are spread over a larger number of units.
Total mixed costs increase as volume increases because mixed costs are composed of both a fixed cost component and a variable cost component. The variable cost component increases with more activity, leading to an overall increase in mixed costs.