The ArcGIS JavaScript API can be loaded as a more compact library for mobile browsers. The compact build, as it is called, packs the bare minimum of the libraries needed to view map applications in a mobile browser. Other modules can be downloaded through Dojo's require() statements, but many will not be preloaded with the initial library.
The ArcGIS compact build contains all the modules necessary to build a web map application. It loads the same as the regular ArcGIS API for JavaScript, using require() and define() statements. It also comes with the most frequently used modules, such as esri/Map and esri/layers/ArcGISDynamicMapServiceLayer, to quickly load your maps while using the least bandwidth.