Zen said:
1/3 of the game is as big/bigger than the full StarCraft I
Starcraft 1 = 10 missions per race, some throwaway non-building missions, and that's it.
Starcraft 2 = ~20-30 missions + optional missions (one race), due to a RPG-lite take on the campaign, that would have been impossible to wrap up in 10 missions and move on to the next race.
Between levels, Starcraft 1 had nothing (but dialog and the occasional cut scene). You have a far deeper meta-game going on between levels in StarCraft 2, that will most likely put it far beyond anything in StarCraft 1. There is branching research, branching unit & building upgrades, optional missions, interaction w/ characters on the ship, money to earn and spend on units and the upgrades, and a choice of optional missions to skip by or do, or possibly even chose between.
Even if you want to argue level for level StarCraft 1 was enjoyable due to the variety of races, looking back on it, there's the completely absent gameplay system which is featured in Wings of Liberty, making the campaign specific to your tastes, and that should make for a much more interesting single player experience.
Lore and story-wise, StarCraft 2 (part 1 of 3), is likely to be longer and larger than StarCraft 1, so the argument falls apart there too. By the looks of it, the actual level design and mission goals in StarCraft 2 will be more varied as well.