In the field of economics, marginal is used as the preferred synonym for 'additional'. A marginal cost is the additional cost of producing another unit; marginal utility is the additional benefit that you get from consuming another unit of a good or service.
The term marginal does not indicate a large or small measure, but is entirely dependent on the process being described. While it is often the case that producing the second widget is much cheaper than producing the first (as you are usually using the same factory and the same workers), this is not necessarily the case. For example, when using limited resources marginal cost may go up -- the marginal cost of figurines carved from elephant ivory is likely to increase for the foreseeable future.
The same is true for marginal utility, despite the commonly cited law of diminishing marginal utility; while usually your 10th slice of pizza provides less utility than your first (meaning that the marginal utility of pizza slices decreases during the course of a meal), in some cases the marginal utility may increase with increased consumption; for example, a family with two Smartphones may get more benefit than a family with one. In this case the marginal utility of Smartphones is likely to increase until the point at which each member of the family has a Smartphone... but then drop suddenly when they buy one in surplus to need.