I was taught O Goshi a few weeks ago, and could not believe how well it worked on my Uke, who is at least 100 pounds heavier than I am. In fact, at one point,my instructor had me keep my Uke on my hip, and do 10 knee bend squats, and it was really easy. Bear in mind, though, footwork and Kuzushi are very important, as always.

Yes half hip is less risky against large opponents, just like in sumo. See Uki goshi, the Japanese name of half hip throw.

Im 175cm

I can try and answer this but perhaps you need to send me video clip of your uchikomi. (Welcome by the way to send, I am on facebook) Usually those that end up on toes are not making enough space to turn into the throw. They step too close to partner, and cannot aim feet parallel either. Try starting the uchikomi further out and stomp step to the top point of an equal triangle for the kuzushi step. (as in video) If you break partners balance forwards and up, and your knee is over your toes on the forwards foot, then you can turn with a slight hop and there will be space. Remember you have to draw opponents balance or they can hip defend your throw. The beauty about stepping to top point of triangle is even if the opponent does hip defence most times your body is in such a good position that the throw works anyway. (But no power in legs if you are on your toes, weight should be on the ball of feet/front of feet but heels just touching the mat, so the foot can spring up in the throw.)

I'll put in pratice your advices tomorrow and see what happens, sensei. Thank you for your reply and for the videos, they have helped me a lot since I started

My understanding is over the fulcrum line of a line through both hip bones.

well thanks.