The OP's assumptions about 7.62 cans and 5.56 cans are incorrect ( OP ). If you look at the specs for certain cans like from Silencerco, going with a dedicated 5.56 can is often merely a consideration about weight or physical size of the can.. The 7.62 cans tend to be a little heavier, although that's not always the case.. But as far as performance, the larger 7.62 cans give better results in DB at the muzzle.. The other difference might be which attachment methods are available for the can or serviceability ...
For example, the Saker ASR 556 can is 16.7 oz, with a rating of about 134.0 db for 5.56
The Saker ASR 762 is 23.4 ounces and has a rating of 129.6 db at the muzzle... Quieter.. but more weight.
Exceptions:
The Omega 30 weighs in at 14.0 oz, but is rated for 130.1 db at the muzzle..
The Hybrid 46 weighs in at 13.8 oz, and is rated for 130.4 db at the muzzle ..
Often the larger can wins; if I was interested in 5.56 and .300 BLK, I'd be looking at the Omega 30 or the Hybrid 46 if quiet is your forte... If you are looking for smaller size at the cost of DB, then look at maybe the Saker ASR 556K .. Then maybe you want a tank (Saker ASR 762) ... Or a good all around can (Omega 30).. If you wanted to try out pistol silencers, but centered more towards rifles ( Hybrid 46 ).
Ratings: Within a 1 DB difference, I would consider db ratings essentially the same... 3 or 4 DB is more significant.. A few points difference- means you'll need an expensive meter to tell the difference.. I don't really compare ratings except if from the same manufacturer... I used Silencerco in this example because they do post db measurements. Some things are measured, like weight, in the lightest possibility; adding an ASR mount to the Hybrid will make the weight go up, for example, over the screw-on 5/8 inch direct thread mount.
Reality is that when you choose a silencer, you are choosing it based upon differing criteria; picking your poison.. I started with Silencerco, so most of my guns have silencerco ASR mounts - which factors in. But your choice starting out is more open; you're choosing based upon noise reduction or rating, weight, size, mounting options available, features, ability to service, multi-caliber, etc.. Where I might choose the Hybrid, the Omega might be more like you need.. I'd also bet that the Saker ASR 762 is built more like a tank, and sometimes, a tank is what someone wants... In the end, there is no wrong choice, just the choice that best fits your needs.. At the time that is... Sometimes, it also means getting a second silencer- because that's what you need ... Or maybe the 5th or 6th...