I work with a smaller guy (maybe 5'-7" / 140 lb.) who actually uses this crazy 18 lb. maul as his only means of splitting, and heats his house with an old smoke dragon. He loaned the maul to me once, and it went thru anything I swung it at, but sure wore me out quick. I also found keeping the orientation difficult, since the handle was round schedule 40 pipe, and I could not feel which way it was pointing. He cauled it a Monster Maul, but it was much larger than what I see sold as a Monster Maul today.
When I got back into splitting wood last year, after many years away from it, I was using the 12 lb. maul for everything. More recently, I've been using the 8-pounder, only grabbing the 12 when I have large or difficult rounds to split. I get better head speed, more precise aim, and less fatigue with the 8-pounder. It might be worth trying the Fiskar's 4-lb., if you like it. Perhaps it has a narrower head, which makes up for the lack of heft?