They considered making them smaller but it "just didn't sit right" according to the PP reply to that question on their forums. The Jacks in the box are the same scale as the Warmachine ones. They're fairly simple models though, they're designed to go together without glue and to have interchangable arms.
I'm not convinced they'll be suitable replacement figures for the Warmachine ones in general. Particularly as it's pretty much a given that the heavy jacks will be made in plastic anyway over the next year or so.
What they may be useful for though (aside from playing Grind) is making Merc Warjacks, as Merc jacks are usually based on old, outdated chassis. Another possible use is for making labour/steamjacks for scenery pieces or as part of a supply train etc.