Is this an actual Welsh phrase or just a fun example for the course?
Not an everyday expression! A proverb. But it does illustrate the particular pattern very well.
If it's not a proverb (or even if it is), surely 'It's a thin dog that's winning the race' should be accepted. (I am reporting back to my friend who knows the dogs better than I do, but can't see what's going on.)
Am i being yn dwpach nag arfer? Could someone spell out what the proverb means, or when you might use it, os gwelwch chi'n dda?! - or does it actually simply mean 'skinny folk do better'?! Diolch!