Difference between network card size and capacity
Those field names were intended for other types of hardware, and mapping them to network interfaces doesn't make that much sense. That said, in this context, "capacity" is the maximum bandwidth of the NIC, while "size" is the current bandwidth.
capacity is the maximum speed of the NIC (in this case, 1Gbit/s).
size is the current connection speed (in this case, 100Mbit/s).