Strictly speaking, the CX300 and the shures are both canals - ear buds sit next to the ear, without going into the canal; think free apple buds.
But some canals fit better than others. I hated the CX300, they always fell out on me too, and they sounded crap to boot.
I've now got some different ones (they're not sold in the UK, before you ask), but the tips are exactly the same size and material as the C300, but they fit (and sound) way better.

I think probably because the driver they sit on is a tad bigger.
That's the deal with canals; no one will let you try them out, so it's a suck it and see...