After watching a lot of Rocky Marciano (an old boxing legend, undefeated, stood 5'10" and a goddamn 67" wingspan my gawd), I became curious of how short people would strategize against much taller/longer-armed people. Let's say someone's opponent is half a foot taller and has 10" longer reach (like what Marciano had to deal with). In boxing, shorter people tend to adjust their style to be skilled at working on the inside, keeping constant pressure, and lots of head movement. In MMA, I just can't imagine it. On outside range, the taller guy has an advantage with straights and kicks. On the inside, they'll just clinch/thai clinch you which they also have an advantage. The only possibility I see is takedowns but then most likely the taller person is fighting off the back foot and will be all ready for it. So? Strategize a scenario.
Im a short and stocky build and i compete at 84 Kilo's, therefore my whole time i seem to contest the longer taller frame I like to faint, roll and throw with short shots, i never really jab with a longer guy, once you have your opponent guessing its a good time to shoot off a strike or a feint A simple combination i have worked extensively for MMA is just a sharp jab followed by a left hook to the body, specifically for longer fighters i find this works this great, the movement has to be dynamic, but to summarise i just find it best to be active and use speed / dynamic movement, if you sit out there with no movement your getting your head jabbed off or legs kicked off
throw in alot of those ultra low takedowns. they expect the mid range double legs etc but not so much ankle picks and the like. (see kimo vs tank or randy vs toney for an example)
There are plenty of examples of small guys successfully competing with and beating taller opponents with bigger reaches, both in the ring and the octagon. Best approach is to watch the videos of those matchups and see for yourself how they went about it. Like all things in combat though, nothing is guaranteed to work all the time.