How are you trying to split it?
I've done tons of big oak like that, fresh stuff, and the fiskars handles it. Just start at the edges, no more than two inches max from the bark, work your way around, get all those pieces with the bark off. Sometimes I even go an inch, just barely inside the outside layer of bark. Just depends. The more they suck, the skinnier the first chunks off the round need to be. Once the bark is off, it makes it a lot easier.
After this first slab/debarking, your "round" is not so round anymore, might be a whatever a twelve sided figure is, something like that, however many cuts you take out of it.
I just don't think overall size has anything to do with it, if you follow that method, I've done over 30 inch diameter chunks so far with mine. OK, granted, none of them more than 16" thick though, what I cut for our heater.
Getting the bark off before the serious splitting commences is the prime way to get the wood chunks to come off easy. The bark acts like an incredibly hard band to keep the wood intact, once it is gone, well...Ya, wood sticks together real good all by itself, but that bark just adds some x-factor more effort needed to bust it up, so get rid of it first. the chunks are all still good firewood, so it don't matter either way. Nice and sorta flat, they stack nice....
Then move in to more normal firewood size chunks by whacking across those points you've made, in a little distance, your choice on size, 4-6" maybe. Just keep doing that, eventually you get down to the heart wood and it can be quartered or more, again, your choice.
I've also been doing some hickory rounds fresh enough they ooze sap, bigger than that, with the same technique. And that's some dense stringy wood.
When doing it by hand with the lighter high speed splitting axes, just remember this lame rhyme: "rounds get split, round and round and round"
You just can't use the fiskars the same way you would a maul or wedges, not on anything really big anyway, just not designed for it. the fiskars is NOT a splitting maul substitute, it is a different beast. I mean, sure, I've done it, just for a hoot. You have to use a lot of whacks and walk your natural crack clean across the round. Then go back and do it again. Eventually you'll tell when to go for it, a coupla good ones right in the middle, it'll pop, but..that's the hard way and a lotta work. I can have the whole thing done in the same time and amount of whacks going round and round and round as just getting it in half like using a maul. I'd rather have all the firewood busted up by then. Maximum busted wood with minimal axe strikes.
Actually, thinking about it, for suck wood I have split a lot of, fresh green poplar gives me the most grief, it's like hard rubber. So I let that stuff sit for awhile until it dries some and then crack it. I don't go out of my way to cut it, but makes decent fall and spring wood and got a lot here, I'll take some standing dead or fresh blowdowns or something with a bad lean aiming for a fence. and as long as I got it, it gets split and burned.
Sweetgum...haven't tried any big ones yet with the fiskars. I cut a lot of it, but small, it just gets stacked as is because it'll all fit in the stove already. I won't cut a big one down on purpose, but next time one falls, I'll try it, and report either success or....having to use the hydraulic.
Oh, I touch up the blade a *lot*, sometimes after each round, no more than four rounds. Just a few fast stokes, first couple hard, then ease off make a "slick" stroke. Makes a big difference, even though it still seems sharp, that little extra fine sharpness seems to give a lot more depth and split. I think there's a reason they ship those things OMG sharp compared to what you see for a factory edge with most other axes, etc.
