The storage capacity of a device is defined by the hardware installed by the manufacturer.
The amount of space the OS uses will vary by device/manufacturer based on the additional apps or features that are added.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Android Central.
Gigabytes of storage or RAM? If storage, total, or available to the user? In which carrier's configuration. (Different carriers may have it partitioned differently, so the user space will be different.)