When I graduated from college, I was 5'5", 140 lbs on a fat day. If we were to get in some sort of no strikes MMA type of contest, I'd put a lot of money on myself. I say no strikes because I have zero experience with that sort of thing. You previously said that you wouldn't box someone like that anyhow.
While I'm not a boxer, I am a wrestler. I was a HS state runner up, and an average D3 college wrestler. I have wrestled against national champs; I have wrestled in national tournaments. I'm by no means a MMA expert, but I know my way around the mat.
Here's what I know:
1. from a fly-weight's perspective, about rolling with bigger opponents: There is a significant speed difference in guys over 170 or so. You big guys looks at us fly-weights, and think, man, that dude is quick...we look at you and think, man, he's slow. If you try to make a move, you are so slow to develop it that we see it coming a mile away, and have already started what ever counter we want before you get there. It's not that hard - when you train against lightning quick fly-weights and then go against someone bigger, everything slows down.
2. because you are bigger, I know you will want to try to use your weight and muscle me - you want to play to your strength. Bull rush me...I dare you. I know it's coming, and because I'm faster than you, I will move. Again, because you aren't as fast and can't change directions quickly (damn inertia), you will end up with your face on the ground and me on your back.
3. in the event that you do get a hold of me, I'm squirmy enough that I guarantee I'll get out. I'm good like that.
I know all this because I have rolled with bigger opponents. We used to do this in practice all the time - break the monotony of rolling with the same guys every day. What I give up in girth and strength, I have in speed, agility, and wiggle.
And, I'm not that good. Someone with MMA experience that includes even a little wrestling will be fine.
Your best strategy would be to keep a distance and box - use your reach as an advantage, and don't let him get in on you; but at the same time don't let him get control of any arm or leg because he will use it against you.
update:Since making this post, I have started taking jiu-jitsu and MMA style kickboxing. I have about 7 or 8 months of jiu-jitsu, and only about 3 or 4 weeks of kickboxing. here's what I know:
*jiu-jitsu isn't really about weight. It just doesn't matter all that much. I'm not saying it doesn't matter at all, but it is a different ball-game than wrestling even.
*having longer limbs can help you or hurt you. You can lock up some submissions easier, but having longer arms makes it easier for someone to lock up on you. If you don't know what you are doing, having a longer reach (i.e. being taller) works against you in terms of submission defense.
* as a short fighter, I know my limitations and how to work within that system. There's a lot more to boxing than I was aware of - I would think that someone without any formal training would get schooled pretty quickly. I know that when I'm working on my feet against a moderately experienced fighter, I feel absolutely silly. Fighting isn't about swinging wildly, it's about how to move, slip punches, how to set up combos, how to transfer momentum, how to punch without leaving your jaw exposed...it's not easy by any means.
* there is a guy in our dojo who is probably
5-6, 135 lbs. high school kid, I think he's a junior. I haven't watched him spar (kickbox) very much, but I have seen him routinely dismantle guys in the 6'2", 200 lb range. Not guys off the street either - I'm talking about guys with about as much experience as I have. The dude is good.
I guess the point I'm trying to make here is that experience counts. Lots of experience + conditioning = easy win for the little guy.