Good question - the line can be pretty hazy; can't it?
In that specific
scenario; I'd call it an attack. This is because the opponent (the great big guy) is standing in an agressive posture (of which of course there are many kinds; the spicific posture could be important as well) saying
he's gonna whup your ass.
This is still in the 'warmup' phase of the encounter; the defensive situation has not yet occurred. Should you let tegatana fly; (whether as atemi or simplyto provoke a response); both legally and realistically you've started it; therefore it's an attack. (Legally; you've become the attacker; therefore the one breaking the law.)
If on the other hand the agressor does
become the attacker (IOW he launches his attack); actively defending means to take the initiative and lead his attack into a position you can deal with. For instance; in the case you mentioned if the attacker starts moving with intent and then
you launch tegatana; it's active defense.