The best way to apply the armbar from mount, or from guard (though it's much more difficult) is to keep your bottom heel pressed into your opponents ribs and your top heel trying to reach for your opponent's ear (it won't touch the ear, but the thought is that this will apply some additional pressure to the head with the calf and cinch up the armbar that much more), which will make sure that you are keeping your legs tight across the torso.
Now that I'm looking at the pictures I posted, I can see that Fedor is not doing that, because he had to adjust the angle of his armbar to account for the way that Coleman's sitting, though it would have been just as easy to pull Coleman down (which he does in the armbar used to finish their other fight) and finish him the way I've suggested.
Keeping the heels as tight to your opponent as you can will keep the legs bent, which will give you the opportunity to apply more pressure with your hips, making the armbar more powerful, as opposed to having your legs out straight, where you can have trouble extending all the way through the technique.
I'm glad you asked for the clarification. Did that help?
Yes, that does help. Thanks! I've also been having a lot of trouble with my arm triangles now that I think about it, care to give me a few pointers on how to improve those? I always set it up and I get it in pretty deep, but it seems like more of a neck crank than anything, which I know is wrong, because when you get a tight arm triangle in they go out really fast. Thanks in advance