1. Since Outlook 2013, there is a new maximum attachment size limit

The limit is 20MB for all attachments.

 

2. Warning, this is an advanced manipulation

Type "regedit" in Windows start menu to open Windows registry.

 

3. Search for these keys, (x.0 = your Outlook version : 16.0 = Office 2016)

HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences 
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\x.0\Outlook\Preferences

 

4. Create a DWORD value if it doesn't already exist (0 = no limit)

Type: DWORD 
Name: MaximumAttachmentSize 
Value: 0

 

5. Exit Windows registry and restart Outlook

Source : Microsoft