Looking at the code, the coolant capacity of a big reactor is defined by the volume of the hull of the reactor. i.e. the volume of the outer dimensions minus the volume of the interior (including fuel rods).
You are right, that seems to be exactly it.
Maybe the problems people run into then is that the Volume of the hull becomes too low compared to the steam/t for VERY large reactors(reactor walls/interior goes down as bigger you make box). So you might eventually run into the problem I mentioned above because the reactor walls does not scale the same as the reactor interior volume(and thereby steam output).
EDIT:
Here is a plot of the "Buffer Volume"/"Total reactor Volume" for Cubic reactors as a function of the side length:
The larger the reactor gets(the more steam it can produce) the less buffer it gains per reactor volume.
My best guess is that there is a turning point(location on the scale is inverse proportional to your skills on building efficient reactors) where the buffer cannot keep up with the steam produced.