Several months ago I wrote a critique of the B&W C5 headphones where I complained about the fit. Well, I was wrong. The C5s can fit me quite well, but (duh) you must use the right eartip.
I have small ear canals. I usually use the smallest tips supplied and then hope they are indeed small enough to fit properly. That's exactly what I did with the C5 earphones, and the fit was not very good. In fact the fit was poor enough to severely compromise the otherwise stellar performance of the C5s.
After the review I put the C5 earphones aside and didn't use them again for a couple of months. The reason I tried them again was my experience with the Shure SE215 earphones, which I reviewed for AvGuide.com. With the Shures I found the largest eartip delivered the best fit. While it's counter-intuitive people with small ear canals should try the largest eartips, with some earbuds such as the Shures (and the C5s) that was what got me the best fit.
After my experience with the Shure S215 earbuds I took out my pair of B&W c5s and tried using the largest soft ear-tip. Much to my delight the largest tip provided an excellent fit, with no leaks. Also with the large ear-tips I could use the C5 earbuds in the gym without worrying about them falling out or needing constant adjustment. The isolation still wasn't quite as good as with my Ultimate Ears Reference Monitors or Etymotic 4Ps, but still more than adequate to make the sounds of the gym's bad stereo and barbell clanking recede into the distant background.
The moral of the story is to always try all the eartips in the fit kit. Don't assume that a particular size and type of tip is the only one to ever use. Depending on an earphone design, the tip that "never works" might be the perfect match for your ears.