Imanari roll.
If that fails, kip up and then pull open guard into a balloon sweep. If that fails, berimbolo.
Basically, act like a very aggressive angry monkey. Works every time. Big people are afraid of monkeys.
Work on techniques that don't rely on going straight into them. The low single is good, as is the sweep single. If your judo is on point that opens up a lot of options. But mostly, unless you're good enough and work on takedowns enough that you already take down people 50-100lbs heavier than you on the regular, you should just work on your takedowns until you do. This question gets asked all the time and some of the people answering just throw their favorite takedown out, even if it's not a good idea. Other people say "just pull guard lul". Yet others give a few good answers, but they disregard the fact that most people asking this question likely lack the timing and skill to hit the takedown against resistance on someone their size, let alone much bigger.
The answer should always be: just keep working your takedowns against partners of all sizes, under competent coaching, and you'll find your own answer.
For me it's seoi nage, collar drags, and sweep singles (and pulling SLX). For other people it might be arm drags to singles, or tomoe nage to tripod sweeps, or whatever. Doesn't matter. Just probably don't plan on it being a blast double or osoto gari, I guess.