Big ranges lead to more targets. The real trick is a combination of being able to log on to a big range, seal kills in a short duration, and log off, and do all this without being too strong to appear invincible, but still be able seal the fights you get into. If you play lot of long logins, your enemy learns to know your strengths, and act accordingly. Short logins over varying times usually leads to more fresh people to kill. Once you've killed someone several times, it gets much harder to nail them again usually, since they learn to either plan for you or gtfo.
Being able to dominate mid levels helps a lot with big kill counts, since you get more fresh targets there, as opposed to hero, where you'll usually be facing the same guys over.