Each mecha module has a maximum Points of Complexity (CMX) of components that can be fitted into them. And, each component has a CMX rating which describes how many CMX it takes up when installed.
In GearHead, a good Mecha Engineering skill can overcome these limitations to a significant degree. A persistent and skilled engineer can easily fit twenty CMX points or more of weapons and propulsion jets into a single armor component rated for only one. Many high-end mecha also come with their components overinstalled.
In GearHead 2, however, there are hard limits to how many CMX you can fit into a particular CMX capacity. One can purchase a talent that raises this limit slightly, but a limit exists nontheless.
The CMX of a module is determined by its size + 1, with exception to bodies/torsos which are (size + 1) x 2. In other words, a module's size is one below its CMX rating, and the size of a mecha can be determined by halving the body's CMX rating and then subtracting one.