Here's a .pdf of the indictment itself:
http://www.orlandosentinel.com/media/acrobat/2008-10/42895889.PDF
From what little I know about how the GJ works, they only hear evidence from the prosecution, and determine what, if any, charges should be filed against an accused. And, from what I read on this board leading up to the day the GJ met, they only hear a fraction of the evidence. Obviously, in this case, they wouldn't have heard about any of the truckloads of evidence found when Caylee was found because she had not been recovered at that point.
Regarding the bolded portion of your post above, that's my point: if all the GJ heard was George saying the trunk smelled, and heard from technicians who said that air samples show that there "could" have been a body in the trunk at some point in time, amongst other less-than-definitive evidence (by the way, I must have missed the part about bruises on Caylee, where'd you hear that?), and the outrageous nanny story, then they would have likely just charged her with aggravated manslaughter or some other lesser charge. But they went all the way with murder one. If you read the indictment .pdf, the wording is actually pretty concise: Casey killed Caylee in a premeditated fashion. For the GJ to "go all the way" if you will, and in such a short amount of time, there has to be something more compelling that they saw than what you and I agree to be flimsy evidence.