This is a very uninformed question, but I've searched elsewhere
without answer... apologies in advance.

I noticed that my PHP-FPM processes are nearly maxing out my system's
RAM, leaving all but about 2% remaining.

www-data 30866 0.0 17.4 281688 93452 ? S Feb23 0:34 /usr/
local/bin/php-cgi --fpm --fpm-config /usr/local/etc/php-
www-data 30867 0.1 18.5 282496 99196 ? S Feb23 0:39 /usr/
local/bin/php-cgi --fpm --fpm-config /usr/local/etc/php-
www-data 30868 0.1 18.4 281500 98384 ? S Feb23 0:36 /usr/
local/bin/php-cgi --fpm --fpm-config /usr/local/etc/php-
www-data 30872 0.1 20.5 285916 109528 ? S Feb23 0:34 /usr/
local/bin/php-cgi --fpm --fpm-config /usr/local/etc/php-
www-data 30873 0.0 17.3 266732 92700 ? S Feb23 0:29 /usr/
local/bin/php-cgi --fpm --fpm-config /usr/local/etc/php-

As you can see, each of the 5 processes is consuming approximately 18%
of my system's memory. Is this by design, or should I be worried that
even when under no load, the processes appear to be hanging on to the
RAM?

I have APC enabled, but it's only using 32 MB of the 80MB I allocated
for it.