I don't know why masterbatrader deleted his posts, because he did ask a good question (what if a person is dealing with a massive weight difference, say 40+lbs); but here's my answer in-case he see's this.
"After a certain level of technique, size doesn't matter so much because you're better at off-balancing, or taking advantage of their movement. It's not like we train with someone of similar weight ALL the time. You train with who is there. I'm 5'8" 128lbs as of last week and work with LEO's a lot. Most of the time, my training partners are extremely fit 160 - 225lbs. My two most dominant training partners lately have been 225lbs, and 265lbs; and they have another guy I work with often who is just above 300lbs and has some prior judo training (he also played football so he's not just a fat-body). My sensei, a former Olympian, and another blackbelt in your dojo are both right at the 200lbs mark. You just train with who you have around you, and you mix it up also so you're used to different body-types.
Most Judo is using your opponents weight and barely any of your own. In fact, I think speed and an upright posture is actually much more important then any type of physical strength or size difference.
That's not to say I'm completely disagreeing with the point your bringing up, you certainly do need to be aware of physical size differences and be more choosy with your throw choice; especially in a street situation where bad throws can put you in a place to get stomped. Using myself as an example, being a skinnier guy with long legs and an even build, I'd use O Soto Gari over a technique like Sumi Otoshi. I also really like De Ashi Barai (like it's arguably my very favorite technique), advancing foot sweep, and I know I can hit this on guys who are 300lb's and charging on me with an intent to do damage, so it's still an option in my wheelhouse; other people may not be so quick to go with those particular throws however. A female, skinnier like myself but shorter, may be better suited to a hip throw or using a clinch to an inside trip.
Sorry went on a bit long, but 90% of the time, you're going to go for your best techniques and you're probably going to land them because especially with Judo, you're used to going straight for them. Judo is a lot faster then jiu-jitsu, you get one, two, maybe three attempts at a throw before someone is thrown. It's much more akin to wrestling in that you are constantly working. Especially in street / nogi situations where you can't use grips to stall out.
I don't mean to come off as condescending when I say this so please don't take it that way, but right now you're kind of thinking of off-balancing as simply pushing or pulling your opponent, using all muscle, but it's so much more then that. You can grab a grip, step back, and that will force them to move, you can pull and it will either pull them, or pull yourself into them. You can move, circle, in and out much like in boxing. They will turn, go flat footed, cross their feet, bring their feet to be too narrow or wide, all in response to your movement. You combine all of this with the actions of the lifting hand the pulling hand to move their weight into position.
If someone hunkers down and doesn't want to move, they've committed and commitment can be as equally deadly for them. Because either you can turn and run, or they have presented a hard, stationary target for you to attack. If they stiff arm, simply grab a grip, and do a bicep curl, which will pull either them into you, or you into them depending on weight.
There are 40+ judo throws, and it's through combining these throws that you set up other throws, and you end up in positions to use certain throws over others. For instance one of my favorite combinations is Ko Uchi Gari, minor inside trip with my left leg on their right, they step back to stabalize, I go to O Uchi Gari, major inside trip with my right leg on their left, they step back and to do so must turn slightly, now I go to O Soto Gari, major outside reap crossing my right leg over and sweeping out their right leg. All their weight is on the leg being swept and so they are very light. You're simply providing enough force to push them far enough back to let their weight do the rest of the work. All three of these throws use the off-balancing and footwork of each other to set up the next throw in sequence. Once your opponent is off-balanced, it's their weight taking them for a ride.
When it comes to self-defense, I am infinitely more comfortable using my Judo over my BJJ. I think that says quite a lot about how I feel about Judo's effectiveness. I really love BJJ, I really respect it, and I'm glad I train it and thanks to it can more then handle myself on the ground, but for that initial contact, you just can't go wrong with a good, hard Judo throw. Judo doesn't advertise it as much, but Judo works against larger opponents standing in the same way BJJ does on the ground. It just takes experience and randori.
I mean go to a gym, and maybe try a free week of Judo. It's one of those things you just can't & don't understand until you do it/have it done on you. :)"