Backstab on larger opponents?

Speed kills.

Also speed is also a plus.

Whoever told you that part was not telling you the truth.... or was really ugly...

a-g

Contrary to popular opinion Size Matters.

And in most skills.

Unless you're Celerity, you really need to put some sort of evidence for your claim, like "I've tested this." For example, I myself have played a number of different thieves, and while there may have been a bit of a difference, it's a negligible one.