A mystery!

I really need to get out and mow the lawn ... so while I get on with it, instead of a written post, let me offer you a mystery instead. I went to take down a bird fat feeder and inside was a snail shell. No occupant. And the shell was huge - too big to fit between the bars or through the hole at the bottom. Maybe it slithered in as a smaller snail and decided to stay: but in that case what on earth had it been living on to grow to that size, where it couldn't escape?
There's been nothing in that feeder for a year ...