That using a can designed for a larger caliber will not be as effective on a smaller caliber would be a logical assumption, but I'm not an expert in suppression theory. It's more than just "tuning" to the volume, it's also got something to do with frequency and harmonics and baffle design and probably a host of other considerations with a dash of magic thrown in. Without knowning anything about it, I could make the following assumptions:

We could assume that since the volume of the 7.62 can is bigger than the 5.56 can it would have more effectiveness. There's some basis for a larger volume being more effective in the information on the websites.

We could also assume that since all the gas trapped in the can has to eventually be expelled it has to exit through the bore. Using bernouli's theorem we know that the smaller the venturi the higher the velocity has to be for a given pressure and volume so a can with a smaller hole (5.56) would have a louder component to the perceived noise due to the escaping gases than one with a larger hole (7.72).

But all this assuming is BS because I don't know JACK about suppressor theory. I do assume that it wouldn't be as effective - now the question is how much less effective would it be. A 3 db difference would have twice (or half depending on which way) the sound energy but it takes around a 10 db difference for a human to notice it. But one of the advertised reasons for using a can is to reduce the potential for hearing loss, so anything that gets the level down to below 120 db or so will accomplish that.

The rifle cans aren't as quiet as the effective pistol cans in the best of circumstances since a large component of the noise is the sonic boom. Since you have heard a difference between a .45 can on a 9mm, does anybody have corresponding experience with rifle calibers?

At $200 each for the federal tax plus the cost of the cans and flash hider adaptors we're talking one third the investment if it worked "well enough".