After steaming for 5 mins, why are live PEI mussels shriveled, but PC's mussels plump?
The President's Choice mussels in sauce are (properly) fully cooked and then frozen in packages for your convenience. I can't find heating instructions for their products but, regardless of which method you use, they should be prepared from frozen with the goal being to heat them - not cook them.

Regarding the live PEI mussels, you are overcooking them. If the meat is shriveled, they are overcooked. You can't just assume that 5 minutes is the ticket for properly cooked mussels. 

While there are different methods of cooking mussels and other shellfish such as grilling or baking, for steaming, there are some tips to help ensure that the mussels are fully cooked but not overdone.


Use a wide base pan so that the mussels are not overcrowded in the pan. Cook in more than one batch if necessary.
Use a proper amount of cooking liquid. (Personally, I usually aim for ~.5" - 1", depending on the size of the shellfish.)
Bring your cooking liquid to a boil, add the mussels to the cooking liquid (no steamer basket), and reduce the heat so that the cooking liquid is simmering - NOT BOILING. You want a gentle steam.
Cover the pan with a clear lid. That will allow you to see when they start to open and monitor the cooking.
If you have uniformly sized mussels they will cook more evenly. So, two methods for finishing:


If the mussels are not uniformly sized and smaller ones are opening before others, pluck them out as they open. When the last ones are open, remove the pan from the heat and put all of the mussels back in the pan and let them sit covered for a minute or two.
If the mussels are uniformly sized and are pretty much opening at the same time, as soon as they are all open, leave the pan covered and remove from the heat. Let them sit for a minute or two in the residual heat and they will be ready to serve.



The reason for letting them sit for a minute or two in the residual heat is that some mussels will open very quickly, ~3 minutes. That typically is not considered enough time for them to be fully cooked. And if they take longer, an extra minute or two in the residual heat is not enough to ruin them.

Last but not least, remember that time can vary. Even with mussels that take longer, it's still a quick process.  So it's important to keep your eye on things.