Unless you're a Sumo wrestler, the JAL First seat is big enough. Size of seat is not a problem with JAL, even if a person with a measuring tape would find the Cathay seat to be wider.
JAL service is very good. They do load plenty of Champagne, and plenty of everything else. The main toilet is very nice (there are two toilets up front but one is clearly the principal toilet for the F pax and the other one is a more general-purpose toilet, maybe as nice as the BA F toilet). The entertainment screen is huge, the bed pad is comfortable (it has a firmer and softer side, but they're basically lovely and comfortable or even softer and enveloping). The food is good and you should have the full Japanese meal, especially ex-HND. The ex-LHR F catering doesn't suck either.
The service from the crew is... excellent. There's just no other way to say it. They operate on the invisible-elves principle - you are offered drinks and the main meal sometime after takeoff (you could, of course, choose to eat later) and after that left alone - unless you press the call button, at which point a crew member will appear within seconds and attend to you until you until it's appropriate to leave you alone again. There's no official second meal service time, so you need to summon food whenever you want. There is an extensive menu of small meals, snacks, etc.
JAL all the way for me.