I hate to be pedantic, but: the size of the salt is directly proportional to the amount of the message you want to consume (because, as you know, in food there's a certain fixed ratio of salt to other ingredients).
So if you want a person to not accept the premise, you should say "take it with a tiny grain of salt", which implies "accept very little of the premise".
> the size of the salt is directly proportional to the amount of the message you want to consume (because, as you know, in food there's a certain fixed ratio of salt to other ingredients).
Hmm, where are you getting this idea from? It seems to me that the salt is meant to act as some sort of antidote and/or preservative:
> The idea comes from the fact that food is more easily swallowed if taken with a small amount of salt. Pliny the Elder translated an ancient antidote for poison with the words 'be taken fasting, plus a grain of salt'.
> Pliny’s Naturalis Historia, 77 A.D. translates thus:
> After the defeat of that mighty monarch, Mithridates, Gnaeus Pompeius found in his private cabinet a recipe for an antidote in his own handwriting; it was to the following effect: Take two dried walnuts, two figs, and twenty leaves of rue; pound them all together, with the addition of a grain of salt; if a person takes this mixture fasting, he will be proof against all poisons for that day.
Fun fact: The word "salary" comes from salt, too– Romans greatly valued the stuff for its preservative qualities, and built roads for its transportation.
"Take it with a grain of salt" is an idiom. An idiom by its very nature has a meaning different from its literal meaning. You'll misunderstand an idiom if you try to reason about it from the literal meaning of the words.
A common example is "I could care less." When you see that phrase in a forum of literal-minded people like this one, someone is likely to reply that they should have said "I couldn't care less." Yes, of course that is what the writer meant - but this doesn't make the idiom wrong. The idiom is sarcasm - saying the opposite of its literal meaning. This is more clear when you hear it spoken out loud: the emphasis is on the words "I" and "care", indicating the sarcastic intent.
Or take the phrase "kicked the bucket", which obviously does not literally mean "died". In the movie It's a Mad Mad Mad Mad World, when Jimmy Durante's character dies after his car flies off the road, there happens to be a bucket next to his foot and in his death spasm he kicks it. What makes the scene funny is the use of the idiom's literal meaning - unlike the normal use of an idiom.
In fact, you might say they turned the idiom on its head. But idioms don't have heads, do they? :-)
The salt accompanies the message when the message should be treated with some amount of skepticism or something like that. If no skepticism is needed, no one mentions taking salt with it.
The amount of salt mentioned then increases with the amount of of skepticism needed.
So, if much more suspicion/skepticism is needed, we see that people mention much more salt.
The salt is like an antidote for the questionable sources, not just a flavoring.
Since this is a nerd forum and I am a nerd I will step up the pedantry by pointing out that a grain is a foundational unit of measurement of mass[0] equivalent to 64.798 milligrams, making discussion of both boulder sized and tiny grains of salt somewhat nonsensical.
So if you want a person to not accept the premise, you should say "take it with a tiny grain of salt", which implies "accept very little of the premise".