Coincidentally, I've been thinking about this exact same thing: Nate Silver deduces that the president-elect is deliberately opening with a lower stimulus number and he intends to allow the overall pricetag to go much, much higher.
Clearly the Republicans are going to gripe about spending (despite massive evidence in favor of spending), so the Republicans will do a whiny pee-pants dance irrespective of the president-elect's base number or the final number. So why not let it go higher? It's a win-win.