I can't really speak to street fights as that is not my experience. I have had some rather memorable experiences with monsters in the dojo, either my own home studio that I operated for about 9 years or as a traveling rogue student. I have a bunch of thoughts on this, not really formed into a coherent structure plus one interesting fight story if I can get the motvation to capture it in writing.
So again, 6'0, 165, pretty much a grappling style mongrel with experience in various flavors of SOMBO, Judo, Japanese Jujitsu, some Brazilian Jujitsu and whatever else I tripped across. Once upon a time I used to travel to a lot of matches of various styles or 'open' formats. So, moderately experienced and in my late 20's-early 40's at the time.
One interesting thing about running a small home studio/club (2X / week, 9 years, 2-16 studients/participants) is that damn near ANYONE (size/skill/background) can and will find you and walk in the door. As the most experienced guy, even my best students would look at the unknown and un-calibrated new guy cock-eyed and go - he's all yours, dude. So I got a lot of interesting experiences, especially as the new guy doesn't usually care as much about people he doesn't (yet) know.
I trained in a bunch of stuff over the years but honestly my first love is sport grappling with a rules set only restricted to miimize injury. My clear preference is for hardest fight possible with a fully resistive opponent - but knowing we all need to go to work in the morning.
While we went through striking phases, my striking game is probably my weakest suit - I can beat non-strikers, survive and fake it briefly with strikers but want to get to the ground with any competent striker ASAP before I get my lights knocked out. My style? Atypical. Standing to take-down (throw if possible) to set up a submission. I much prefer the scramble to set position / chess methodology that BJJ favors. Submissions, arms, legs, best yet strangles from anywhere, gi or no-gi, I don't care. I hve few 'favorite' moves, I just try to find the thing that my opponent is weakest at and get to work.
So...big guys. Monsters in particular. If striking is in play the first rule for me is to stay conscious and minimize injury. Never believe 'light contact' with big guys. Even if they claim to be pulling their punches (they lie) there is just too much momentum. A single decent hit and it's probably game over so I tend to play striking very conservatively and hook up early.
Once hooked up somehow, next is the standing game. Things are dicey here as well but unless they get a clean throw you are less likely to be taken out. I defend throws well enough, and can take a fall well but still...
One recurring challenge - I have been snatched and literally picked up overhead at least a half a dozen times by different monsters. The question here is how you come down. If they have good control of you and simply slam you from shoulder+ height you are done. That said, there are things that can be done up there. First, stay connected by grabbing whatever you can. The, straighten, arch and extend horizontally - they can't keep you up. Or get 'wiggly' and become the ultimate nightmare free weight. As you inevitably start to unexpectedly come down, switch to a cannonball posture and create gravitationally-assisted rotational energy, transferring the energy into the monster - accelerating him with a 'sacrifice' type throw as you decelerate your fall.
The more common risk is that you get mounted or otherwise controlled bya. Dude that is simply too strong/massive to 'technique' your way out of. All I can say is - don't go there or very quickly do whatever you need to do to get out if you find yourself there. Or you are done.
One key thing is simply to stay alive. Big guys almost inevitably consume a LOT more O2 than smaller and hopefully more aerobially/anerobically efficient guys. Get them burning hot and fast and try to keep them in that state as long as possible. Time can be your friend here. The simplest of control positions should be maintained as long as possible. You can't rush things - let the monster wear himself down.
Also, monsters tend to be sloppy on technique (WARNING: that doesn't mean they don't KNOW the techniques) and rely on (did I mention that physics here sucks) size / strength / mass and the disparity of this with respect to their opponents. Why? Because they cana nd it just works. They tend to under-estimate smaller opponents and/or over-estimate the speed with which they can end the fight.
EVERY little thing needs to be played to your advantage...or the conclusion is predictable.
-more to come-