Misunderstanding $=

I've tried to use a recipe like this to report the size of a mail: 

#Toss huge things
#
:0
 * > 500000
{
   LOG="Huge mail received: $=
`formail -XFrom`
"
   LOGABSTRACT=all
   :0
   $TRASH
}

but the size logged is always zero.  I'm obviously misunderstanding
the passage in the procmailrc manpage that says:

"BUGS
       The only substitutions of environment variables  that  can
       be  handled  by  procmail  itself  are  of the type ... $=;
       whereby ... and $= will contain the score of the last recipe."

The recipe above does indeed toss huge things, so I thought I'd
be able to get the "score of the last recipe."  What am I missing?

TIA

Jim
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