Are you sure it's not a restriction ?
Your attachments may be under the gateway's limit taken piece by piece, but it may not be the case when you attach all of them.
Replace the server & domain parts below, and have a look.

#!/usr/bin/perl -w
use strict;
use Net::SMTP;
use IO::Scalar;
my $smtp;
my $debug = '';
tie *STDERR, 'IO::Scalar', \$debug;

$smtp = Net::SMTP->new('your.smtp.server',
                           Hello => 'a.domain.it.accepts',
                           Timeout => 30,
                           Debug   => 1,
                          );
$smtp->quit();
untie *STDERR;
print grep {/SIZE/} (split(/\n/,$debug));
[download]

Note: the STDERR redirection comes from this node, and if you look at MIME::Lite's docs, you'll notice MIME::Lite->send accepts a "sub" facility, where you could plug a variant of the snippet, to "enable" debugging. Take a look at send_by_smtp's source for further inspiration on how to do it.
Update: Actually, calling MIME::Lite->send_by_smtp with Debug => 1 as part of it's args may be simpler (haven't tried)