size to MB

I am having a problem with a script.

If I do

get-mailboxstatistics -id xxx| select itemcount, totalitemsize

I get the correct answers (i.e. no of items and the mailbox size in KB)

I want to display the size in MB so I tried

get-mailboxstatistics -id xxx| select itemcount, @{label="MB";expression={$_.totalitemsize.value.tomb()}}

But I get a blank value and can't work out why. I have read quite a few different posts and it looks like I ave the right syntax but not the right result

Any ideas?

TIA

Andy