Is there a significant difference in Memory settings between 9.5 and 12

Upgraded from 9.5 to 12 and 12 would not start with the current configured
Shared Buffers.

Same hardware, same config file.

Which tells me something has changed, are there new default settings in the
12 postgresql.conf file that are not being called out in my 9.5 config file
that could be the cause or?

9.5
shared_buffers 10GB

12
shared_buffers 5GB

12 will not start at 10GB, even though it's the same hardware, same config
file, same physical box, same everything, just version 12 vs 9.5

So this tells me that maybe i'm missing a new memory setting in 12 , that
is not being overwritten in my local config file (I run an include and my
own settings), this worked fine in 9.5 but I'm guessing there is something
considerably different between 9.5 and 12 (I'm just not seeing it). Anyone
with insight into what memory settings may have been added in 10/11/12 that
are significantly different than 9.5?

Thanks
Tory

CentOS 7
Postgresql 9.5.x
Postgresql 12.2