To be fair, your average new-grad / entry level job at MS is probably not going to be one of those positions building complex stuff, it's going to be "cog in the system" type of work. It's dubious whether you'll even get to interact with engineers that you might learn something from. There was a story recently about this, and is also corroborated by pretty much all of my friends who ended up there.
Basically, if you're at the point where you think you can do some architectural work and real decision making, you're likelier to be allowed to do that at a startup than at a large company.
new grads probably shouldn't be making 'architectural work' decisions, at least not on their own, and doing it 'on their own' is more likely in a 'breakout startup' situation.
Basically, if you're at the point where you think you can do some architectural work and real decision making, you're likelier to be allowed to do that at a startup than at a large company.