I'd have to guess that Barrowman has the bigger piece, and is quite fond of getting it out,
If you search for David Tennant under the search facility here, you'll find the pictures that were posted a few weeks ago, he isn't as Billie Piper suggested "David Ten-inch"...but look for yourself.
There are Barrowman threads here too, though apparently he has a reputation for being ...."Legs Up Lucy" I think is the direct quote....
But he is a very handsome man, who cares. Not me!!!!
(Though Torchwood was complete pants- total arse gravy television!)