DO-214 is a standard that specifies a group of semiconductor packages for surface mounted diodes.
The standard includes multiple variants such as
DO-214AA (SMB), DO-214AB (SMC), and DO-214AC (SMA).
DO-214AC (SMA) is the smallest,
DO-214AB (SMC) is the largest.
DO-214AC (SMA) < DO-214AA (SMB) < DO-214AB (SMC)
从以上的图片看来，这三种封装看上去差不多，没什么区别，但从实物体积上来分，可以看出：
SMA < SMB < SMC.