"unset HISTFILESIZE" not working in .bashrc

[perryh]

Configuration Information [Automatically generated, do not change]:
Machine: i386
OS: freebsd6.1
Compiler: cc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i386' 
-DCONF_OSTYPE='freebsd6.1' -DCONF_MACHTYPE='i386-portbld-freebsd6.1' 
-DCONF_VENDOR='portbld' -DLOCALEDIR='/usr/local/share/locale' -DPACKAGE='bash' 
-DSHELL  -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib  -I/usr/local/include 
-O2 -fno-strict-aliasing -pipe
uname output: FreeBSD fbsd61 6.1-RELEASE FreeBSD 6.1-RELEASE #30: Mon Jan  1 
23:01:34 PST 2007     perryh@fbsd61:/usr/src/sys/i386/compile/GENERIC  i386
Machine Type: i386-portbld-freebsd6.1

Bash Version: 3.1
Patch Level: 10
Release Status: release

Description:

I'm not sure whether this is a bug, or something that I don't
understand.

I added these lines to ~/.bashrc:

echo this is .bashrc
unset HISTFILESIZE

and started a new xterm.  It did report "this is .bashrc" before
giving the first prompt, so I know that it did read the new
~/.bashrc, but it seems that either the "unset HISTFILESIZE" was
ignored or something set it to the default value after ~/.bashrc
had been read.  I then entered ". .bashrc" so that .bashrc would
be read again, and this time the unset worked as expected.

Should I be using some other method to unset HISTFILESIZE?

The xterm shows:

this is .bashrc
[perryh@fbsd61 /usr/home/perryh]$ echo $HISTFILESIZE
500
[perryh@fbsd61 /usr/home/perryh]$ . .bashrc
this is .bashrc
[perryh@fbsd61 /usr/home/perryh]$ echo $HISTFILESIZE

[perryh@fbsd61 /usr/home/perryh]$ bash --version
GNU bash, version 3.1.10(1)-release (i386-portbld-freebsd6.1)
Copyright (C) 2005 Free Software Foundation, Inc.