That's a dilemma most of us would like to have. Unless your a fish cutting ninja filleting a 100 lb tuna while it's in your lap would be about impossible as well as dangerous.
As for stuffing meat into your fish bag until it's full and throwing the rest away; please dont do that. The waste is wrong and if you are checked by a warden and have1/2 a tuna you're gonna get a ticket for wastage. And you'd still have to fillet the fish across your lap.
The best solution I can come up with is to lighten the fish as much as possible while remaining legal. Removing the head, guts, and fins would really reduce the length and weight of the fish. There would still be some knife work but most of these cuts can be done away from you.
Basically do the first 3 steps this fish ninja does
https://youtu.be/aeiI1T7uu0o. Mike