You're right, length (in terms of proper pacing) can have an affect on how well a game plays, and perhaps I should have made that clearer. For instance, if Half-Life 1 ended a third of the way though, as you were running along a corridor, or met that big snake-like green beast thing, it would be so jarring and out of place with the rest of the game that it would have a negative impact.
The point I was making, though, was that reviewers do not seem to be concerned even with this detail. They take length as an absolute, regardless of pacing and regardless of the proper time to stop, and use that number to judge the game. And that, I think, is wrong.