On Windows Vista and 7, Windows Defender is only protect against spyware. Complete protection by M$ is provided separately by MSE.
On Windows 8 and later, Windows Defender is a full antivirus and antimalware suite. This version is also called Windows Defender Antivirus.
Apparently, the downloaded Windows Defender definition is small one (mpas-fe.exe), which appears to be the definition update for Windows Vista and 7. The definition update for Windows Defender for W8 and later (mpam-fe.exe) is not downloaded, i.e. not present in the client\wddefs\x64-glb folder. More information can be obtained from the WDSI portal [1].
It would be better to provide definition for Windows Defender Antivirus in addition to legacy Windows Defender when the checkbox "Include Windows Defender definition" is selected.
Legacy Windows Defender and MSE update file name is mpas-fe.exe
Windows Defender Antivirus definition update file name is mpam-fe.exe
[1] https://www.microsoft.com/en-us/wdsi/definitions