message_size_limit, queue_minfree, and mail spool not on root directory

My mail spool is not on my root directory:

data_directory = /mnt/xvdb/var/lib/postfix
mail_spool_directory = /mnt/xvdb/var/spool/mail
queue_directory = /mnt/xvdb/var/spool/postfix
virtual_mailbox_base = /mnt/xvdb/var/spool/mail

However, it seems that the capacity of my root mount has some bearing
on the evaluation of Postfix's message_size_limit and queue_minfree. I
am getting "insufficient system storage" errors despite having enough
space in /mnt/xvdb. I have much less space available on /.

I found some relevant functions in Postfix: fsspace() and
smtpd_check_queue(). I also wrote a program confirming my understanding
of fsspace()'s use of statfs().

Before I continue diving into Postfix's source code, can someone provide
some information about the details surrounding message_size_limit and
queue_minfree? Is my assumption valid that the capacity of / should be
irrelevant when my spool is on /mnt/xvdb? Is there another directory I
should be concerned about other than the four listed above?