Going slightly OT, but my cat's breed is apparently renowned for good weight management. He knows when to stop and often leaves food behind which he'll come back to later on in the day. Other's would just scoff it all down!
My cat is over 8kg and I can assure you that he is in excellent condition. He is a former street tom cat, who was extremely emaciated when we got him and he was still nearly 7kg when skin and bones. Like people, all cats are different.
My former cat was a heffalump and my Maine Coon is big. My other cat was a wisp. They do vary. My former boy cat did overshoot the tray when it was just normal sized, that's why I got the jumbo ones, but he also had some bladder issues from time to time. My big girl doesn't miss, I think it's more a boy thing. Is your cat a bengal? That pattern reminds me of a bengal.
Aldi is selling Conni bed pads on Sat 7 April. If puss is still overshooting, getting some to use under his tray might be an option. (depends whether you prefer washing the floor or doing laundry, I suppose.)
One of our guys does the exact same thing. We just use big bunnings storage crates as litter boxes. We started off cutting out a u shape to help them get in and out, but now just provide a step of some sort for them to jump in and out. Usually a smaller storage box with the lid on and something inside to weight it for stability. It's still annoying because by going in the corner the clumping litter doesn't work as well. But it stays in the box. We don't have room for it now, but at our old house we had two of these Ikea wardrobes with the litter crates inside. We jigsaw'd holes in the sides for entry and had a 4x4 expedit in between (with one vertical divider removed) as the entry step. Completely hidden, reduced litter tracking, helped contain smells, easy to access to scoop regularly.
