I think its easier to whip up a big tree with a 5/8 or bigger line. The added weight gives the rope more momentum so it whips around the tree better. I just made a 15ft lanyard from 9.5mm line and love it, but if I was going to gaff up a 70inch conifer, I'ed use one of my home made 5/8 inch three strand lanyards with a cable in it. I don't think that 9.5 mm could be made to whip around a tree that big.
So a lot has to do with your climbing Technic.
I would second that and definitely recommend a 5/8 for much larger trees. A rope lanyard also gets stuck on cut branch collars way more easily than a wire core when flicking it up.
Rope lanyard is ok if you have nothing else but you will be farting around with it way more when spiking. Wire core is way faster and easier when spiking up a larger tree.
Most everything comes down to a good technique and knowing what to use for certain situations. The same piece of equipment just used with a better and more suitable technique can be the difference between you loving it or wanting to throw it out of the tree! Experience and training counts for everything. Having the most flashy gear might look good, but if your technique and training plain sucks you are not going to be having a great time up there.
Someone told me this long ago. An well practiced and expertly trained golfer can win the US masters with a $100 set of second hand clubs. A hack with no patience couldn't win with the best and most expensive clubs in the world.