On Long And Short Arguments, Vatican Saying 26, And Lucretius

Today I used VS 26 “One must presume that long and short arguments contribute to the same end,” in a post on Facebook, but in using it in a context I thought might be amusing, I realized that this is one that always seemed a little dark to me.  It doesn’t really make much sense as worded, and it’s not even clear to me whether it means “be concise” or whether it has some deeper meaning.   In looking for an explanation, I came across a statement in Lucretius that may well have been drawn from the same source.  And as an added bonus for this post, I get to point out the benefits of comparing multiple translations:

My favorite “poetic” translation of Lucretius is by Rolfe Humphries, who in Book IV includes these lines:

How swiftly is an image borne along?
What speed is given its flight across the air?
How long a space, how brief a time, is used
As each with different aim pursues its course?
In answering, I’ll try to have my verse
Be sweetly-spoken, but not long; I’ll take
Swans for my model, not the honking cranes
Raucous in flight among the southern clouds.

That seems to be relevant, but not entirely clear either.  Why choose swans as a model rather than cranes?

But let’s look to Munro, who I gather aimed for a more literal translation:

Now mark: how swift the motion is with which idols are borne along, and what velocity is assigned to them as they glide through the air, so that but a short hour is spent on a journey through long space, whatever the spot towards which they go with a movement of varied tendency, all this I will tell in sweetly worded rather than in many verses; as the short song of the swan is better than the loud noise of cranes scattered abroad amid the ethereal clouds of the south.

So if Lucretius was drawing on the same thought that produced Fragment 26, the oddly worded “one must presume” should not obscure the point:  fewer words are indeed better when the same purpose can be achieved.  Of course that begs the question of whether there is something even deeper.  Are cranes and swans both calling out for the same reason?  Presumably the point is hammered home if we know that both cranes and swans are accomplishing the same thing through their calling, but that swans are achieving their goals more efficiently.  (But I thought cranes were migratory and using their calls to help them fly in flocks over long distances.  Is that the same reason that swans sing?)