Those tobacco tins are round. The traditional SAS tobacco tin is rectangular. For example,
this BCB tin is the larger-sized one. The large ones are about 4.4"x3.2"x1.1" and the small ones 3.25"x2.6"x0.9". My Altoids tin is 3.75"x2.5"x0.75". These are external dimensions with the lid on.
The Altoids tin is half an inch longer than the small tobacco but slightly thinner and less deep - it's a slightly more convenient shape for the same capacity. If you take the lid off, they are both about the same height. With lid closed the tobacco tin is higher mostly because it has a seal that is more or less air- and water-proof, which can be handy. The Altoids lid is hinged, and the hinge is made by piercing the body, so it is not at all water-proof, but is less likely to come open if the tin is over-stuffed. (Some people like to seal the tins with tape, but I prefer to keep mine accessible for casual dipping-into.)
Even though they are made in England (or at least mine were), I found Altoids quite hard to locate here. No supermarket had them. I eventually found them in a small petrol station shop. Tobacco tins were much easier to find.
Tobacco tins have several potential advantages over a plastic pouch:
+ Rigid, better protection for their contents.
+ Water proof, ditto.
+ The metal surface can be used as a (rather poor) signalling mirror.
+ Fire proof - the larger tin holds nearly 0.2l, which is nearly enough to be useful for boiling water.
+ Conductors, so protect contents against Electro-Magnetic Pulse.