Big arpeggios - I have to move my whole arm from the shoulder and do what seems to me to be really extreme elbow movements. sometimes I wonder if I am going to take my own eye out..
I think this is the way for long, fast arpeggios. Your arm is the carrier, setting your fingers free to do what they have to do. Also, sloping the wrist in the right direction is important. It seems to be less "thumb under or fingers over" than moving the arm/hand and aiming.
I'm dogged by smaller hands as I like to attempt LIszt. It means more broken chords and even missing out some chord notes if I have to. His Study "Ricordanza" gave me a fair few problems as there are two passages of fairly quiet repeated chords that cover a tenth (which I can just about reach with certain black note combinations, like Bb to Db) in my left hand, not so in my right.
As for octaves I had to practice a lot away from the keyboard.
Incidentally, Liszt was an excellent teacher. He taught me how to suspend melodies between the thumbs of both hands which has been particularly useful in my "lounge" playing. You can make it sound like three hands at work!