wow really? you used a product called big bud and it's not causing big buds? but it has a cartoon graphic on the front, right?
Okay, jokes aside... It's 3 weeks. rate of development varies, but this is normal for 3 weeks and you really can't be sure of bud size at this point.
If it stops adding new pistils/calyxes, then you can fret over an average yielding plant.
pistil turning brown is way too early -- is a fan directly pointed at it? was it pollinated? you should be investigating for a cause and hope it's environmental or a 1-off oddity.
if it was a selfed plant, expect it to be a little less robust than its parent... so if the parent has a meh yielder, this will be even less than that.