In the case of desktop computers, there are many motherboard form factors. The most common ones are ATX and Micro-ATX. They allow for optimized airflow within the PC tower as well as easy access to components to install cables and connectors effortlessly. The difference between these two form factors lies in their dimensions: the Micro-ATX form factor generally has about half the number of PCI connectors as the ATX form factor, usually between 2 or 3 PCI slots. This is more than sufficient when there is no need to add additional cards. The question becomes more complex in the case of laptops. There is no standard in this area, as managing space within the chassis is a significant market challenge.
Thus, each manufacturer will have its own motherboard form factor that it can change (to create a lighter PC or conversely a more comprehensive one with numerous connectors).