Your math is right, but the web page isn't. I think the Global Settings page is wrong. It is not kb.
Global settings shows 50000kb:
[root@mail bin]# ./zmprov gcf zimbraMtaMaxMessageSize
zimbraMtaMaxMessageSize: 51200000
[root@mail bin]# grep message_size_limit /opt/zimbra/postfix/conf/main.cf
message_size_limit = 51200000
Global settings shows 100000kb:
[root@mail bin]# ./zmprov gcf zimbraMtaMaxMessageSize
zimbraMtaMaxMessageSize: 102400000
[root@mail bin]# grep message_size_limit /opt/zimbra/postfix/conf/main.cf
message_size_limit = 102400000
From postfix.org site:
message_size_limit (default: 10240000)
The maximal size in bytes of a message, including envelope information.
From Zimbra site: Configuring maxmessagesize - Zimbra :: Wiki
And, zimbraFileUploadMaxSize is no longer used for mail:
"Until ZCS 5, there is a separate configuration parameter for file uploads."
Of course, the General Information tab of General Settings still shows it, but its use has changed:
Bug 27610 – Use zimbraMtaMaxMessageSize exclusively for mail, and use zimbraFileUploadMaxSize for docs/briefcase
Turns out, it now only applies to Docs/Briefcase, not mail messages:
"Add zimbraFileUploadMaxSize to GlobalConfig -> General since it is not applied to MTA but docs/briefcase"
I tried to check whether the docs/briefcase max (apparently in SOAP) was set in kb, but didn't have any luck. After many years, and hundreds of applications, I do not recall a single one which set sizes in kb.
