Not always the case, some bitches (greyhounds in particular) don't swell a great deal.
They swell enough to notice.
I agree with mackay here, we have a bitch that doesn't swell or puff up at all. The only sign of a heat is the blood, this bitch was mated once and it was a natural mating, the stud dog knew his job well. She produced 7 pups. I understand what your saying colliegrey but there are exceptions to every rule.