My smallest is a Webley & Scott 32-40. I love the small doubles and probably 90% of all my hunting is done with either this rifle or the first double I ever owned, an Alex. Henry 360-2 1/4. The Henry is by far my favorite and has accounted now for in excess of 300 head of game. I believe a small double makes you really hunt because you don't have any power in reserve.
By using a small cal. double in thick cover on pigs, where your shots and visibility are less then 10 metres, you expose yourself to the potential of untold adventure. It's just a big game hunt scaled down.
My 'big' double these days is a 9.3x74R. I use it if I'm feeling lazy or the job at hand is deemed more serious.

Underlever.