It's not merely size. It's size-relative-to-market.
Canada negotiates with drugs suppliers. Supplier says "take a hike". Canada says, "OK, you're out of the market", turns to next drug supplier.
Canada here has right-of-refusal, and can negotiate, as a single block entity, with ability to restrict market, to each. The drugs company which finds those terms acceptable has a market.
A bloc of 42 million in the US is only about 12% of the total market. The discussion there still leaves another 290M possible customers for a drugs vendor. Hence the strength is with the vendors.
If US Medicare had rights to exclusively negotiate drugs deals (it doesn't), this would change tremendously. Similarly for single payor.
Which is precisely why the entire medical services industry is so mortally opposed to it. They'd lose absolutely all market leverage. They know this.
