In a smaller system, the design of the entire system can be shown in one place.
The design of bigger systems needs to be done/shown at multiple levels.
Multi-level design can be done in a top-down manner, bottom-up manner, or as a mix.
Agile design can be contrasted with full upfront design in the following way:
Agile designs are emergent, they’re not defined up front. Your overall system design will emerge over time, evolving to fulfill new requirements and take advantage of new technologies as appropriate. Although you will often do some initial architectural modeling at the very beginning of a project, this will be just enough to get your team going. This approach does not produce a fully documented set of models in place before you may begin coding. -- adapted from agilemodeling.com